Method of manufacturing semiconductor device

ABSTRACT

In a manufacturing process of a transistor including an oxide semiconductor film, oxygen doping treatment is performed on the oxide semiconductor film, and then heat treatment is performed on the oxide semiconductor film and an aluminum oxide film provided over the oxide semiconductor film. Consequently, an oxide semiconductor film which includes a region containing more oxygen than a stoichiometric composition is formed. The transistor formed using the oxide semiconductor film can have high reliability because the amount of change in the threshold voltage of the transistor by a bias-temperature stress test (BT test) is reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method ofmanufacturing the semiconductor device.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and an electronicappliance are all semiconductor devices.

2. Description of the Related Art

A technique by which transistors are formed using semiconductor thinfilms over a substrate having an insulating surface has been attractingattention. Such transistors are applied to a wide range of electronicdevices such as an integrated circuit (IC) and an image display device(display device). A silicon-based semiconductor material is widely knownas a material for a semiconductor thin film applicable to a transistor.As another material, an oxide semiconductor has been attractingattention.

For example, a transistor whose active layer is formed using anamorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) andhaving an electron carrier concentration lower than 10¹⁸/cm³ isdisclosed (see Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528

SUMMARY OF THE INVENTION

However, in a semiconductor device including an oxide semiconductor, anoxygen vacancy in the oxide semiconductor might change the electricconductivity thereof. Such a phenomenon becomes a factor of fluctuationin the electric characteristics of a transistor formed using the oxidesemiconductor.

In view of the above problem, an object is to provide a semiconductordevice formed using an oxide semiconductor, which has stable electriccharacteristics and high reliability.

In a manufacturing process of a transistor including an oxidesemiconductor film, oxygen doping treatment is performed on the oxidesemiconductor film, and then heat treatment is performed on the oxidesemiconductor film and an aluminum oxide film provided over the oxidesemiconductor film. Consequently, an oxide semiconductor film whichincludes a region containing more oxygen than a stoichiometriccomposition is formed. Further, it is possible to remove an impuritycontaining a hydrogen atom by performing heat treatment on the oxidesemiconductor film. More specifically, for example, the followingmanufacturing method can be employed.

One embodiment of the present invention is a method of manufacturing asemiconductor device, including the steps of forming a silicon oxidefilm, forming an oxide semiconductor film in contact with the siliconoxide film, forming an aluminum oxide film over the oxide semiconductorfilm, forming a region containing more oxygen than a stoichiometriccomposition in the oxide semiconductor film by performing oxygen dopingtreatment on the oxide semiconductor film so that oxygen is supplied tothe oxide semiconductor film, and performing heat treatment on the oxidesemiconductor film having been supplied with oxygen and the aluminumoxide film.

Another embodiment of the present invention is a method of manufacturinga semiconductor device, including the steps of forming a silicon oxidefilm, forming an oxide semiconductor film in contact with the siliconoxide film, removing a hydrogen atom in the oxide semiconductor film byperforming first heat treatment on the oxide semiconductor film, forminga region containing more oxygen than a stoichiometric composition in theoxide semiconductor film by performing oxygen doping treatment on theoxide semiconductor film so that oxygen is supplied to the oxidesemiconductor film, forming an aluminum oxide film over the oxidesemiconductor film, and performing second heat treatment.

In the above method of manufacturing a semiconductor device, the siliconoxide film and the oxide semiconductor film are preferably successivelyformed in this order without exposure to the air.

In the above method of manufacturing a semiconductor device, a peak of aconcentration of oxygen in the oxide semiconductor film, which isintroduced by the oxygen doping treatment, is preferably higher than orequal to 1×10¹⁸/cm³ and lower than or equal to 3×10²¹/cm³.

In the above method of manufacturing a semiconductor device, an oxideinsulating film may be formed between the oxide semiconductor film andthe aluminum oxide film.

In a manufacturing process of a transistor including an oxidesemiconductor film, oxygen doping treatment is performed, and then heattreatment is performed in a state where an aluminum oxide film which hasa function of preventing entry of water (including hydrogen) into theoxide semiconductor film and a function of preventing release of oxygenfrom the oxide semiconductor film is provided. Consequently, at leastone region where oxygen exists at an amount exceeding that in thestoichiometric composition of the oxide semiconductor film (such aregion is also referred to as oxygen-excess region) can be provided inthe inside (bulk) of the oxide semiconductor film or an interfacebetween an insulating film and the oxide semiconductor film. Note thatoxygen which is added by the oxygen doping treatment may exist betweenlattices of the oxide semiconductor.

It is preferable that dehydration or dehydrogenation by heat treatmentbe performed on the oxide semiconductor film and a hydrogen atom or animpurity containing a hydrogen atom such as water in the oxidesemiconductor film be removed, so that the oxide semiconductor film ishighly purified. It is preferable that the amount of oxygen added by theoxygen doping treatment be set to be greater than that of hydrogen inthe highly purified oxide semiconductor film which has been subjected tothe dehydration or dehydrogenation.

Note that the above “oxygen doping treatment” means that oxygen (whichincludes at least one of an oxygen radical, an oxygen atom, and anoxygen ion) is added to a bulk. Note that the term “bulk” is used inorder to clarify that oxygen is added not only to a surface of a thinfilm but also to the inside of the thin film. In addition, “oxygendoping treatment” includes “oxygen ion implantation treatment” and“oxygen plasma doping” in which oxygen that is made to be plasma isadded to a bulk.

The effect of the above structure which is one embodiment of theinvention disclosed herein can be easily understood as follows. Notethat the description below is just one consideration.

In general, oxygen contained in an oxide semiconductor film dynamicallyrepeats reactions of bonding to and detachment from metal elements inthe oxide semiconductor as shown by the following formula (I). Since themetal element from which oxygen is detached has a dangling bond, anoxygen vacancy exists in the portion of the oxide semiconductor film,from which oxygen is detached.[Formula 1]M−O

M+O  (1)

An oxide semiconductor film according to one embodiment of the inventiondisclosed herein contains excess oxygen (preferably at an amountexceeding that in the stoichiometric composition), and thus such anoxygen vacancy can be immediately compensated. Therefore, the density ofstates (DOS) attributed to oxygen vacancies in the film can be reduced.For example, assuming that the average DOS in the case where the amountof oxygen contained in the oxide semiconductor film agrees with that inthe stoichiometric composition is approximately higher than or equal to10¹⁸ cm⁻³ and lower than or equal to 10¹⁹ cm⁻³, the average DOS of anoxide semiconductor containing more oxygen than the stoichiometriccomposition can be approximately higher than or equal to 10¹⁵ cm³ andlower than or equal to 10¹⁶ cm⁻³.

In addition, it has been confirmed that as the thickness of the oxidesemiconductor film is increased, fluctuation in the threshold voltage ofa transistor tends to increase. It can be guessed that this is becausean oxygen defect in the oxide semiconductor film is one cause of thechange of the threshold voltage and increases in number as the thicknessof the oxide semiconductor film is increased. As described above, in thetransistor according to one embodiment of the invention disclosedherein, since the amount of oxygen contained in the oxide semiconductorfilm is increased by the oxygen doping treatment, an oxygen defect inthe film which is caused by the dynamic reaction according to the aboveformula (I) can be immediately compensated. Accordingly, the transistoraccording to one embodiment of the invention disclosed herein can haveless fluctuation in threshold voltage, because time for formation of adonor level due to an oxygen defect can be shortened and the donorlevels can be substantially removed.

Excess oxygen is contained in an oxide semiconductor film and analuminum oxide film is provided over the oxide semiconductor film so asto prevent oxygen from being released, whereby it is possible to preventgeneration and increase of a defect in the inside of the oxidesemiconductor and interfaces between the oxide semiconductor and layerswhich are over/under and in contact with the oxide semiconductor. Inother words, the excess oxygen contained in the oxide semiconductor filmfunctions to fill an oxygen vacancy defect. Accordingly, a semiconductordevice having stable electric characteristics and high reliability canbe provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a plan view and cross-sectional views whichillustrate one embodiment of a semiconductor device;

FIGS. 2A to 2D illustrate one embodiment of a method of manufacturing asemiconductor device;

FIGS. 3A to 3C are a plan view and cross-sectional views whichillustrate one embodiment of a semiconductor device;

FIGS. 4A to 4C are a plan view and cross-sectional views whichillustrate one embodiment of a semiconductor device;

FIGS. 5A to 5D illustrate one embodiment of a method of manufacturing asemiconductor device;

FIGS. 6A to 6C are a plan view and cross-sectional views whichillustrate one embodiment of a semiconductor device;

FIGS. 7A to 7C illustrate one embodiment of a semiconductor device;

FIGS. 8A to 8C illustrate embodiments of a semiconductor device;

FIG. 9 illustrates one embodiment of a semiconductor device;

FIG. 10 illustrates one embodiment of a semiconductor device;

FIG. 11 illustrates one embodiment of a semiconductor device;

FIGS. 12A and 12B illustrate one embodiment of a semiconductor device;

FIGS. 13A to 13F illustrate electronic appliances;

FIGS. 14A1, 14A2, 14B1, and 14B2 show results of SIMS measurement;

FIGS. 15A1, 15A2, 15B1, and 15B2 show results of SIMS measurement;

FIGS. 16A to 16D show results of TDS measurement; and

FIGS. 17A to 17D show results of TDS measurement.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and an example of the invention disclosed inthis specification will be described in detail with reference to theaccompanying drawings. Note that the invention disclosed in thisspecification is not limited to the following description, and it iseasily understood by those skilled in the art that modes and details canbe variously changed. Therefore, the invention disclosed in thisspecification is not construed as being limited to the description ofthe following embodiments and example.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify theinvention.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and amethod of manufacturing the semiconductor device will be described withreference to FIGS. 1A to 1C, FIGS. 2A to 2D, and FIGS. 3A to 3C. In thisembodiment, a transistor including an oxide semiconductor film will bedescribed as an example of the semiconductor device.

FIGS. 1A to 1C are a plan view and cross-sectional views whichillustrate a bottom-gate transistor 410 as an example of a semiconductordevice. FIG. 1A is a plan view, and FIGS. 1B and 1C are cross-sectionalviews along line A-B and line C-D in FIG. 1A, respectively. Note that inFIG. 1A, some components of the transistor 410 (e.g., an insulating film407) are omitted for brevity.

The transistor 410 illustrated in FIGS. 1A to 1C includes, over asubstrate 400 having an insulating surface, a gate electrode layer 401,a gate insulating film 402, an oxide semiconductor film 403, a sourceelectrode layer 405 a, a drain electrode layer 405 b, and the insulatingfilm 407.

In the transistor 410 illustrated in FIGS. 1A to 1C, the oxidesemiconductor film 403 has been subjected to oxygen doping treatment andincludes an oxygen-excess region. By performing oxygen doping treatment,a sufficient amount of oxygen to compensate an oxygen vacancy in theoxide semiconductor film 403 can be contained therein, so that thetransistor 410 can have higher reliability.

Further, an aluminum oxide film is provided as the insulating film 407.Since aluminum oxide has a barrier property and is less likely totransmit moisture, oxygen, and another impurity, entry of an impuritysuch as moisture from the outside after completion of the device can beprevented. Moreover, release of oxygen from the oxide semiconductor film403 can be prevented. Note that the insulating film 407 preferablyincludes an oxygen-excess region.

It is preferable that the gate insulating film 402 also include anoxygen-excess region for the following reason. When the gate insulatingfilm 402 includes an oxygen-excess region, oxygen can be prevented frommoving from the oxide semiconductor film 403 to the gate insulating film402, and oxygen can be supplied from the gate insulating film 402 to theoxide semiconductor film 403.

An insulator may further be provided over the transistor 410. In orderto electrically connect the source electrode layer 405 a or the drainelectrode layer 405 b to a wiring, an opening may be formed in the gateinsulating film 402 or the like. A second gate electrode may further beprovided above the oxide semiconductor film 403. Note that the oxidesemiconductor film 403 may be processed into an island shape.

FIGS. 2A to 2D illustrate an example of a method of manufacturing thetransistor 410.

First, a conductive film is formed over the substrate 400 having aninsulating surface and subjected to a photolithography step, so that thegate electrode layer 401 is formed. Note that a resist mask may beformed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask; thus, manufacturing cost can be reduced.

There is no particular limitation on a substrate that can be used as thesubstrate 400 having an insulating surface as long as it has heatresistance high enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. A single crystalsemiconductor substrate or a polycrystalline semiconductor substrate ofsilicon, carbon silicon, or the like; a compound semiconductor substrateof silicon germanium or the like; an SOI substrate; or the like can beused as the substrate 400. Any of these substrates provided with asemiconductor element may be used as the substrate 400.

A flexible substrate may be used as the substrate 400. In the case ofusing a flexible substrate, a transistor including an oxidesemiconductor film may be directly formed over the flexible substrate,or a transistor including an oxide semiconductor film may be formed overa different manufacturing substrate and then separated to be transferredto the flexible substrate. Note that in order to separate the transistorfrom the manufacturing substrate and transfer it to the flexiblesubstrate, a separation layer may be provided between the manufacturingsubstrate and the transistor including the oxide semiconductor film.

An insulating film serving as a base film may be provided between thesubstrate 400 and the gate electrode layer 401. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 400, and can be formed to have a single-layer structure or astacked-layer structure with the use of one or more films selected froma silicon nitride film, a silicon oxide film, a silicon nitride oxidefilm, and a silicon oxynitride film.

The gate electrode layer 401 can be formed by a plasma CVD method, asputtering method, or the like to have a single-layer structure or astacked-layer structure with the use of a metal material such asmolybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium,or scandium, or an alloy material containing any of these as a maincomponent.

Next, the gate insulating film 402 is formed over the gate electrodelayer 401. In this embodiment, a silicon oxide film is formed as thegate insulating film 402 by a plasma CVD method, a sputtering method, orthe like. The gate insulating film 402 may have a stacked-layerstructure including the silicon oxide film and a film containing siliconnitride, silicon oxynitride, silicon nitride oxide, aluminum oxide,aluminum nitride, aluminum oxynitride, aluminum nitride oxide, hafniumoxide, gallium oxide, or a mixed material of any of these. Note that thesilicon oxide film is preferably in contact with the oxide semiconductorfilm 403 formed later.

A high-density plasma CVD method using microwaves (e.g., with afrequency of 2.45 GHz) is preferably employed for forming the gateinsulating film 402 because an insulating layer can be dense and canhave high breakdown voltage and high quality. This is because when ahighly purified oxide semiconductor is closely in contact with thehigh-quality gate insulating film, the interface state density can bereduced and favorable interface characteristics can be obtained.

Further, as the gate insulating film, an insulating layer whose filmquality and interface characteristics with the oxide semiconductor areimproved by heat treatment performed after film formation may be used.In any case, any insulating film can be used as long as film quality asa gate insulating film is high, the interface state density with theoxide semiconductor is reduced, and a favorable interface can be formed.

The gate insulating film 402 preferably includes an oxygen-excess regionbecause an oxygen vacancy in the oxide semiconductor film 403 can becompensated by excess oxygen contained in the gate insulating film 402.

Next, over the gate insulating film 402, the oxide semiconductor film403 having a thickness greater than or equal to 2 nm and less than orequal to 200 nm, preferably greater than or equal to 5 nm and less thanor equal to 30 nm is formed (see FIG. 2A).

The oxide semiconductor film 403 may be formed using a metal oxidematerial containing two or more kinds selected from In, Ga, Zn, and Sn.For example, a four-component metal oxide such as an In—Sn—Ga—Zn—O-basedmaterial; a three-component metal oxide such as an In—Ga—Zn—O-basedmaterial, an In—Sn—Zn—O-based material, an In—Al—Zn—O-based material, aSn—Ga—Zn—O-based material, an Al—Ga—Zn—O-based material, or aSn—Al—Zn—O-based material; a two-component metal oxide such as anIn—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-basedmaterial, a Zn—Mg—O-based material, a Sn—Mg—O-based material, anIn—Mg—O-based material, or an In—Ga—O-based material; an In—O-basedmaterial; a Sn—O-based material; or a Zn—O-based material may be used.In addition, any of the above oxide semiconductors may contain anelement other than In, Ga, Sn, and Zn, for example, SiO₂.

Here, for example, an In—Ga—Zn—O-based oxide semiconductor means anoxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn)and there is no particular limitation on the composition ratio thereof.

As the oxide semiconductor film 403, a thin film formed using a materialexpressed by a chemical formula of InMO₃(ZnO)_(m) (m>0) can be used.Here, M represents one or more metal elements selected from Zn, Ga, Al,Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, or Ga andCo.

In the case where an In—Zn—O-based material is used as an oxidesemiconductor, a target to be used has a composition ratio of In:Zn=50:1to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably In:Zn=15:1 to 1.5:1 in an atomicratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a targetused for formation of an In—Zn—O-based oxide semiconductor which has anatomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

The oxide semiconductor film is in a single crystal state, apolycrystalline (also referred to as polycrystal) state, an amorphousstate, or the like.

As the oxide semiconductor film 403, a c-axis aligned crystalline oxidesemiconductor (CAAC-OS) film may be used.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystalline portions andamorphous portions are included in an amorphous phase. Note that in mostcases, the crystalline portion fits inside a cube whose one side is lessthan 100 nm. From an observation image obtained with a transmissionelectron microscope (TEM), a boundary between an amorphous portion and acrystalline portion in the CAAC-OS film is not clear. Further, with theTEM, a grain boundary in the CAAC-OS film is not found. Thus, in theCAAC-OS film, a reduction in electron mobility, due to the grainboundary, is suppressed.

In each of the crystalline portions included in the CAAC-OS film, ac-axis is aligned in a direction parallel to a normal vector of asurface where the CAAC-OS film is formed or a normal vector of a surfaceof the CAAC-OS film, triangular or hexagonal atomic arrangement which isseen from the direction perpendicular to the a-b plane is formed, andmetal atoms are arranged in a layered manner or metal atoms and oxygenatoms are arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystalline portions, thedirections of the a-axis and the b-axis of one crystalline portion maybe different from those of another crystalline portion. In thisspecification, a simple term “perpendicular” includes a range from 85°to 95°. In addition, a simple term “parallel” includes a range from −5°to 5°.

In the CAAC-OS film, distribution of crystalline portions is notnecessarily uniform. For example, in the formation process of theCAAC-OS film, in the case where crystal growth occurs from a surfaceside of the oxide semiconductor film, the proportion of crystallineportions in the vicinity of the surface of the oxide semiconductor filmis higher than that in the vicinity of the surface where the oxidesemiconductor film is formed in some cases. Further, when an impurity isadded to the CAAC-OS film, the crystalline portion in a region to whichthe impurity is added becomes amorphous in some cases.

Since the c-axes of the crystalline portions included in the CAAC-OSfilm are aligned in the direction parallel to a normal vector of asurface where the CAAC-OS film is formed or a normal vector of a surfaceof the CAAC-OS film, the directions of the c-axes may be different fromeach other depending on the shape of the CAAC-OS film (thecross-sectional shape of the surface where the CAAC-OS film is formed orthe cross-sectional shape of the surface of the CAAC-OS film). Note thatwhen the CAAC-OS film is formed, the direction of c-axis of thecrystalline portion is the direction parallel to a normal vector of thesurface where the CAAC-OS film is formed or a normal vector of thesurface of the CAAC-OS film. The crystalline portion is formed by filmformation or by performing treatment for crystal growth such as heattreatment after film formation.

With the use of the CAAC-OS film in a transistor, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light can be reduced. Thus, the transistor has highreliability.

Note that nitrogen may be substituted for part of oxygen contained inthe CAAC-OS film.

The oxide semiconductor film 403 is formed by a sputtering method, amolecular beam epitaxy method, an atomic layer deposition method, or apulsed laser deposition method. Here, the oxide semiconductor film 403can be formed by a sputtering method.

In order to form a CAAC-OS film as the oxide semiconductor film 403, thesubstrate 400 is heated during formation of the oxide semiconductor film403. The temperature at which the substrate 400 is heated is higher thanor equal to 150° C. and lower than or equal to 450° C.; the substratetemperature is preferably higher than or equal to 200° C. and lower thanor equal to 350° C. When the substrate is heated at a high temperatureduring formation of the oxide semiconductor film, a CAAC-OS film inwhich the proportion of a crystalline portion is higher than theproportion of an amorphous portion can be formed.

In the formation of the oxide semiconductor film 403 by a sputteringmethod, the concentration of hydrogen contained in the oxidesemiconductor film 403 is preferably reduced as much as possible. Inorder to reduce the hydrogen concentration, a high-purity rare gas(typically argon), high-purity oxygen, or a high-purity mixed gas of arare gas and oxygen, from which an impurity such as hydrogen, water, ahydroxyl group, or hydride is removed, is supplied into a treatmentchamber of a sputtering apparatus as an atmosphere gas, as appropriate.Further, the treatment chamber may be evacuated with the use of acryopump having high capability in evacuating water and a sputter ionpump having high capability in evacuating hydrogen in combination.

The gate insulating film 402 and the oxide semiconductor film 403 may besuccessively formed without exposure to the air. For example, thefollowing may be performed: impurities containing hydrogen attached to asurface of the gate electrode layer 401 provided over the substrate 400are removed by heat treatment or plasma treatment, and then the gateinsulating film 402 and the oxide semiconductor film 403 aresuccessively formed in this order without exposure to the air. In thismanner, impurities containing hydrogen attached to the surface of thegate electrode layer 401 can be reduced, and an atmospheric componentcan be prevented from being attached to an interface between the gateelectrode layer 401 and the gate insulating film 402 and an interfacebetween the gate insulating film 402 and the oxide semiconductor film403. As a result, it is possible to manufacture the transistor 410having favorable electric characteristics and high reliability.

After the oxide semiconductor film 403 is formed, heat treatment (firstheat treatment) is preferably performed on the oxide semiconductor film403. Excessive hydrogen (including water and hydroxyl) in the oxidesemiconductor film 403 can be removed by the first heat treatment.Moreover, excessive hydrogen (including water and a hydroxyl group) inthe gate insulating film 402 can also be removed by the first heattreatment. The first heat treatment is performed at a temperature higherthan or equal to 250° C. and lower than or equal to 700° C., preferablyhigher than or equal to 450° C. and lower than or equal to 600° C. orlower than the strain point of the substrate.

The heat treatment can be performed in such a manner that, for example,an object to be processed is introduced into an electric furnace inwhich a resistance heating element or the like is used and heated at450° C. for 1 hour in a nitrogen atmosphere. During the heat treatment,the oxide semiconductor film 403 is not exposed to the air to prevententry of water and hydrogen.

A heat treatment apparatus is not limited to an electric furnace, andmay an apparatus for heating an object to be processed by heatconduction or heat radiation from a medium such as a heated gas. Forexample, a rapid thermal annealing (RTA) apparatus such as a gas rapidthermal annealing (GRTA) apparatus or a lamp rapid thermal annealing(LRTA) apparatus can be used. An LRTA apparatus is an apparatus forheating an object to be processed by radiation of light (electromagneticwave) emitted from a lamp such as a halogen lamp, a metal halide lamp, axenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or ahigh pressure mercury lamp. A GRTA apparatus is an apparatus forperforming heat treatment using a high-temperature gas. As the gas, aninert gas which does not react with an object to be processed by heattreatment, such as nitrogen or a rare gas such as argon, is used.

For example, as the first heat treatment, GRTA treatment may beperformed as follows. An object to be processed is put in an inert gasatmosphere that has been heated, heated for several minutes, and takenout from the inert gas atmosphere. The GRTA treatment enableshigh-temperature heat treatment for a short time. Moreover, the GRTAtreatment can be employed even when the temperature exceeds the uppertemperature limit of the object to be processed. Note that the inert gasmay be switched to a gas containing oxygen during the treatment. This isbecause defect levels in the energy gap due to oxygen vacancies can bereduced by performing the first heat treatment in an atmospherecontaining oxygen.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (such as helium, neon, or argon) as a maincomponent and does not contain water, hydrogen, or the like ispreferably used. For example, the purity of nitrogen or a rare gas suchas helium, neon, or argon introduced into the heat treatment apparatusis higher than or equal to 6N (99.9999%), preferably higher than orequal to 7N (99.99999%) (that is, the impurity concentration is lowerthan or equal to 1 ppm, preferably lower than or equal to 0.1 ppm).

The above heat treatment (first heat treatment) can be referred to asdehydration treatment, dehydrogenation treatment, or the like because ofits effect of removing hydrogen, water, and the like. The dehydrationtreatment or the dehydrogenation treatment can be performed, forexample, after oxygen doping treatment. Such dehydration treatment ordehydrogenation treatment may be performed once or plural times.

Next, a conductive film which is to be the source electrode layer andthe drain electrode layer (including a wiring formed in the same layeras the source electrode layer and the drain electrode layer) is formedover the oxide semiconductor film 403 and is processed, so that thesource electrode layer 405 a and the drain electrode layer 405 b areformed (see FIG. 2B).

The conductive film used for the source electrode layer 405 a and thedrain electrode layer 405 b is formed using a material that canwithstand a heat treatment step performed later. For example, a metalfilm containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W,or a metal nitride film containing any of the above elements as acomponent (a titanium nitride film, a molybdenum nitride film, or atungsten nitride film) can be used. Alternatively, a film of ahigh-melting-point metal such as Ti, Mo, or W or a metal nitride film ofany of these elements (a titanium nitride film, a molybdenum nitridefilm, or a tungsten nitride film) may be stacked on one of or both abottom side and a top side of a metal film of Al, Cu, or the like.Further alternatively, the conductive film used for the source electrodelayer and the drain electrode layer may be formed using a conductivemetal oxide. As the conductive metal oxide, indium oxide (In₂O₃), tinoxide (SnO₂), zinc oxide (ZnO), indium oxide-tin oxide (In₂O₃—SnO₂,abbreviated to ITO), indium oxide-zinc oxide (In₂O₃—ZnO), or any ofthese metal oxide materials containing silicon oxide can be used.

Note that it is preferable that etching conditions be optimized so asnot to etch and divide the oxide semiconductor film 403 when theconductive film is etched. However, it is difficult to obtain etchingconditions in which only the conductive film is etched and the oxidesemiconductor film 403 is not etched at all. In some cases, part of theoxide semiconductor film 403 is etched off through the etching of theconductive film, so that an oxide semiconductor film having a groove(depressed portion) is formed.

Next, the insulating film 407 which covers the source electrode layer405 a and the drain electrode layer 405 b and is in contact with part ofthe oxide semiconductor film 403 is formed.

An aluminum oxide film can be used as the insulating film 407. Aluminumoxide has a barrier property and is less likely to transmit moisture,oxygen, and another impurity. Therefore, by providing an aluminum oxidefilm over the oxide semiconductor film 403, the aluminum oxide filmfunctions as a passivation film and prevents an impurity such asmoisture from entering the oxide semiconductor film 403 from the outsideafter completion of the device. Moreover, release of oxygen from theoxide semiconductor film 403 can be prevented.

The insulating film 407 can be formed to a thickness of at least 1 nm bya method by which an impurity such as water or hydrogen does not enterthe insulating film 407, such as a sputtering method, as appropriate.When hydrogen is contained in the insulating film 407, entry of hydrogeninto the oxide semiconductor film or extraction of oxygen from the oxidesemiconductor film by hydrogen is caused; thus, a back channel of theoxide semiconductor film might have lower resistance (n-typeconductivity) and a parasitic channel might be formed. Therefore, it isimportant that a film formation method in which hydrogen is not used isemployed in order to form the insulating film 407 containing as littlehydrogen as possible.

It is preferable that a high-purity gas from which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed be used as asputtering gas used for formation of the insulating film 407.

Note that the insulating film 407 includes at least an aluminum oxidefilm and can have a stacked-layer structure including the aluminum oxidefilm and a film containing another inorganic insulating material.

Next, the oxide semiconductor film 403 is subjected to oxygen dopingtreatment, so that an oxygen-excess region is formed (see FIG. 2C). Bythe oxygen doping treatment, oxygen 421 is supplied to the oxidesemiconductor film 403, so that oxygen is excessively contained in atleast one of an interface between the insulating film 407 and the oxidesemiconductor film 403, the inside of the oxide semiconductor film 403,and the interface between the oxide semiconductor film 403 and the gateinsulating film 402. Formation of the oxygen-excess region in the oxidesemiconductor film 403 enables an oxygen vacancy to be immediatelycompensated. Accordingly, charge trapping centers in the oxidesemiconductor film 403 can be reduced.

The oxygen doping treatment is performed so that the amount of oxygencontained in the oxide semiconductor film 403 exceeds that in thestoichiometric composition of the oxide semiconductor film 403. Forexample, a peak of the concentration of oxygen in the oxidesemiconductor film 403, which is introduced by the oxygen dopingtreatment, is preferably higher than or equal to 1×10¹⁸/cm³ and lowerthan or equal to 3×10²¹/cm³. The oxygen 421 for the doping includes anoxygen radical, an oxygen atom, and/or an oxygen ion. Note that theoxygen-excess region may exist in part (including the interface) of theoxide semiconductor film.

In the oxide semiconductor, oxygen is one of main component materials.Thus, it is difficult to accurately estimate the oxygen concentration ofthe oxide semiconductor film by a method such as secondary ion massspectrometry (SIMS). In other words, it can be said that it is hard todetermine whether oxygen is intentionally added to the oxidesemiconductor film.

It is known that isotopes such as ¹⁷O and ¹⁸O exist in oxygen and thatthe proportions of ¹⁷O and ¹⁸O in all of the oxygen atoms in nature areapproximately 0.037% and approximately 0.204%, respectively. That is tosay, it is possible to measure the concentrations of these isotopes inthe oxide semiconductor film by a method such as SIMS; therefore, theoxygen concentration of the oxide semiconductor film may be able to beestimated more accurately by measuring the concentration of such anisotope. Thus, the concentration of the isotope may be measured todetermine whether oxygen is intentionally added to the oxidesemiconductor film.

Part of the oxygen 421 added to (contained in) the oxide semiconductorfilm may have a dangling bond of oxygen in the oxide semiconductor. Thisis because such a dangling bond is bonded to hydrogen remaining in thefilm so that hydrogen can be fixed (made to be an immovable ion).

Oxygen for the doping (an oxygen radical, an oxygen atom, and/or anoxygen ion) may be supplied from a plasma generating apparatus with theuse of a gas containing oxygen or from an ozone generating apparatus.More specifically, for example, the oxygen 421 can be generated with anapparatus for etching treatment on a semiconductor device, an apparatusfor ashing on a resist mask, or the like to perform oxygen dopingtreatment on the oxide semiconductor film 403.

The oxygen doping treatment may be performed on the oxide semiconductorfilm 403 at any timing after the oxide semiconductor film 403 is formed,for example, before the source electrode layer 405 a and the drainelectrode layer 405 b are formed.

After the oxygen doping treatment, heat treatment (preferably secondheat treatment) is performed. The heat treatment is preferably performedat a temperature higher than or equal to 350° C. and lower than or equalto 650° C., further preferably higher than or equal to 450° C. and lowerthan or equal to 650° C. or lower than the strain point of thesubstrate. The heat treatment may be performed in an atmosphere ofnitrogen, oxygen, ultra-dry air (air in which the water content is lessthan or equal to 20 ppm, preferably less than or equal to 1 ppm, furtherpreferably less than or equal to 10 ppb), or a rare gas (such as argonor helium). Note that it is preferable that water, hydrogen, or the likebe not contained in the atmosphere of nitrogen, oxygen, ultra-dry air, arare gas, or the like. The purity of nitrogen, oxygen, or a rare gaswhich is introduced into a heat treatment apparatus is preferably set tobe higher than or equal to 6N (99.9999%), further preferably higher thanor equal to 7N (99.99999%) (that is, the impurity concentration is lowerthan or equal to 1 ppm, preferably lower than or equal to 0.1 ppm).

Through the above steps, the oxide semiconductor film 403 in whichformation of an oxygen vacancy is suppressed can be formed. By thesecond heat treatment, oxygen, which is one of main component materialsof the oxide semiconductor, may be supplied from the gate insulatingfilm 402 that is an insulating film containing oxygen to the oxidesemiconductor film 403. In the case where a CAAC-OS film is used as theoxide semiconductor film 403, a crystal structure included in the oxidesemiconductor film 403 might be disordered by the oxygen dopingtreatment to be an amorphous structure; however, crystallinity can berecovered by performing heat treatment after the oxygen dopingtreatment.

Although the timing of the oxygen doping treatment and the timing of theheat treatment (second heat treatment) performed after the oxygen dopingtreatment are not limited to those in this embodiment, the heattreatment needs to be performed at least after the insulating film 407is formed. This is because, since the aluminum oxide film used as theinsulating film 407 has a high blocking effect and thus is less likelyto transmit both oxygen and an impurity such as hydrogen or moisture,release of oxygen from the oxide semiconductor film 403 can be preventedby performing the heat treatment after the insulating film 407 isformed.

The dehydration treatment or the dehydrogenation treatment and theoxygen doping treatment are performed on the oxide semiconductor film,whereby the oxide semiconductor film 403 can be highly purified so asnot to contain impurities that are not main components thereof as muchas possible and can be an i-type (intrinsic) or substantially i-typeoxide semiconductor film. The highly purified oxide semiconductor film403 contains extremely few (close to zero) carriers derived from adonor, and the carrier concentration thereof is lower than 1×10¹⁴/cm³,preferably lower than 1×10¹²/cm³, further preferably lower than1×10¹¹/cm³.

Through the above steps, the transistor 410 is formed (see FIG. 2D). Inthe transistor 410, an oxygen-excess region is formed by the oxygendoping treatment, whereby formation of an oxygen vacancy in the insideof the oxide semiconductor film or the interface thereof can besuppressed and the number of donor levels in the energy gap due tooxygen vacancies can be reduced or the donor levels can be substantiallyremoved. Further, when oxygen is supplied to the oxide semiconductorfilm 403 by the oxygen doping treatment or the heat treatment after theoxygen doping treatment, an oxygen vacancy in the oxide semiconductorfilm 403 can be compensated. Moreover, a hydrogen ion remaining in theoxide semiconductor film 403 can be fixed by the supplied oxygen.Therefore, fluctuation in the electric characteristics of the transistor410 is suppressed, and the transistor 410 is electrically stable.

Further, the transistor 410 is preferably subjected to heat treatmentfor dehydration or dehydrogenation, and can be a transistor includingthe oxide semiconductor film 403 from which an impurity such ashydrogen, water, hydroxyl, or hydride (also referred to as hydrogencompound) is intentionally removed by the heat treatment.

FIGS. 3A to 3C illustrate another structural example of a transistoraccording to this embodiment. FIG. 3A is a plan view of a transistor420, and FIGS. 3B and 3C are cross-sectional views along line E-F andline G-H in FIG. 3A, respectively. Note that in FIG. 3A, some componentsof the transistor 420 (e.g., the insulating film 407) are omitted forbrevity.

As in the case of the transistor 410 illustrated in FIGS. 1A to 1C, thetransistor 420 illustrated in FIGS. 3A to 3C includes, over thesubstrate 400 having an insulating surface, the gate electrode layer401, the gate insulating film 402, the oxide semiconductor film 403, thesource electrode layer 405 a, the drain electrode layer 405 b, and theinsulating film 407.

One of differences between the transistor 420 illustrated in FIGS. 3A to3C and the transistor 410 illustrated in FIGS. 1A to 1C is the stackingorder of the source electrode layer 405 a and the drain electrode layer405 b, and the oxide semiconductor film 403. That is, the transistor 420includes the source electrode layer 405 a and the drain electrode layer405 b which are in contact with the gate insulating film 402, and theoxide semiconductor film 403 which is provided over the source electrodelayer 405 a and the drain electrode layer 405 b and is at least partlyin contact with the gate insulating film 402. Description of thetransistor 410 can be referred to for the details.

In the case of employing the structure illustrated in FIGS. 3A to 3C, aneffect similar to that obtained in the case of employing the structureillustrated in FIGS. 1A to 1C can be obtained.

In the transistor described in this embodiment, when the amount ofoxygen contained in the oxide semiconductor film is increased by oxygendoping treatment, degradation due to electrical bias stress or thermalstress can be suppressed and degradation due to light can be reduced.Further, an oxygen-excess region is formed in the oxide semiconductorfilm by the oxygen doping treatment, whereby an oxygen vacancy in thefilm can be compensated. Furthermore, an impurity containing a hydrogenatom such as hydrogen, water, a hydroxyl group, or hydride (alsoreferred to as hydrogen compound) is removed from the oxidesemiconductor by heat treatment, whereby the oxide semiconductor filmcan be highly purified and become i-type (intrinsic). Inclusion of suchan oxide semiconductor film enables the transistor to have lessfluctuation in electric characteristics such as threshold voltage and tobe electrically stable.

As described above, according to this embodiment, a semiconductor devicewhich is formed using an oxide semiconductor and has stable electriccharacteristics can be provided. Further, a semiconductor device havinghigh reliability can be provided.

The methods, structures, and the like described in this embodiment canbe combined as appropriate with any of the methods, structures, and thelike described in the other embodiments.

Embodiment 2

In this embodiment, another embodiment of a semiconductor device and amethod of manufacturing the semiconductor device will be described withreference to FIGS. 4A to 4C, FIGS. 5A to 5D, and FIGS. 6A to 6C. Notethat the same portions as Embodiment 1 or portions having functionssimilar to those of Embodiment 1 can be formed as in Embodiment 1, andalso the same steps as Embodiment 1 or steps similar to those ofEmbodiment 1 can be performed as in Embodiment 1; therefore, repetitivedescription thereof is omitted. In addition, detailed description of thesame portions is not repeated.

FIGS. 4A to 4C are a plan view and cross-sectional views whichillustrate a top-gate transistor 510 as an example of a semiconductordevice. FIG. 4A is a plan view, and FIGS. 4B and 4C are cross-sectionalviews along line I-J and line K-L in FIG. 4A, respectively. Note that inFIG. 4A, some components of the transistor 510 (e.g., the insulatingfilm 407) are omitted for brevity.

The transistor 510 illustrated in FIGS. 4A to 4C includes, over thesubstrate 400 having an insulating surface, a base insulating film 506,the oxide semiconductor film 403, the source electrode layer 405 a, thedrain electrode layer 405 b, a gate insulating film 502, the gateelectrode layer 401, and the insulating film 407.

In the transistor 510 illustrated in FIGS. 4A to 4C, at least one of thebase insulating film 506 and the gate insulating film 502 preferablyincludes an oxygen-excess region for the following reason. When aninsulating film in contact with the oxide semiconductor film 403includes an oxygen-excess region, oxygen can be prevented from movingfrom the oxide semiconductor film 403 to the insulating film in contactwith the oxide semiconductor film 403, and oxygen can be supplied fromthe insulating film in contact with the oxide semiconductor film 403 tothe oxide semiconductor film 403.

FIGS. 5A to 5D illustrate an example of a method of manufacturing thetransistor 510.

First, the base insulating film 506 is formed over the substrate 400having an insulating surface, and then the oxide semiconductor film 403is formed in contact with the base insulating film 506 (see FIG. 5A).Note that after the oxide semiconductor film 403 is formed, heattreatment (first heat treatment) is preferably performed on the oxidesemiconductor film 403.

In this embodiment, a silicon oxide film is formed as the baseinsulating film 506 by a plasma CVD method, a sputtering method, or thelike. The base insulating film 506 may have a stacked-layer structureincluding the silicon oxide film and a film containing silicon nitride,silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminumnitride, aluminum oxynitride, aluminum nitride oxide, hafnium oxide,gallium oxide, or a mixed material of any of these. Note that thesilicon oxide film is preferably in contact with the oxide semiconductorfilm 403 formed later.

The base insulating film 506 preferably includes an oxygen-excess regionbecause an oxygen vacancy in the oxide semiconductor film 403 can becompensated by excess oxygen contained in the base insulating film 506.For formation of the oxygen-excess region in the base insulating film506, for example, film formation may be performed in an oxygenatmosphere or a mixed atmosphere containing oxygen and a rare gas.Alternatively, heat treatment may be performed in an oxygen atmosphere.

The base insulating film 506 and the oxide semiconductor film 403 may besuccessively formed without exposure to the air. For example, thefollowing may be performed: impurities containing hydrogen attached to asurface of the substrate 400 are removed by heat treatment or plasmatreatment, and then the base insulating film 506 and the oxidesemiconductor film 403 are successively formed in this order withoutexposure to the air. In this manner, impurities containing hydrogenattached to the surface of the substrate 400 can be reduced, and anatmospheric component can be prevented from being attached to aninterface between the base insulating film 506 and the oxidesemiconductor film 403. As a result, it is possible to manufacture thetransistor 510 having favorable electric characteristics and highreliability.

Next, in a manner similar to that of the step illustrated in FIG. 2B, aconductive film which is to be the source electrode layer and the drainelectrode layer (including a wiring formed in the same layer as thesource electrode layer and the drain electrode layer) is formed over theoxide semiconductor film 403 and is processed, so that the sourceelectrode layer 405 a and the drain electrode layer 405 b are formed(see FIG. 5B).

Next, the gate insulating film 502 which covers the source electrodelayer 405 a and the drain electrode layer 405 b and is in contact withpart of the oxide semiconductor film 403 is formed.

The gate insulating film 502 can be formed by a CVD method, a sputteringmethod, or the like. The gate insulating film 502 is preferably formedso as to contain silicon oxide, silicon nitride, silicon oxynitride,silicon nitride oxide, tantalum oxide, hafnium oxide, yttrium oxide,hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium aluminate(HfAl_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added,hafnium aluminate to which nitrogen is added, or the like. The gateinsulating film 502 may have a single-layer structure or a stacked-layerstructure including a combination of any of the above materials. Thereis no particular limitation on the thickness of the gate insulating film502; in the case where the semiconductor device is miniaturized, thethickness is preferably small in order to ensure operation of thetransistor. For example, in the case where silicon oxide is used, thethickness can be greater than or equal to 1 nm and less than or equal to100 nm, preferably greater than or equal to 10 nm and less than or equalto 50 nm.

When the gate insulating film is thin as in the above description, aproblem of gate leakage due to a tunnel effect or the like is caused. Inorder to solve the problem of gate leakage, the gate insulating film 502may be formed using a high dielectric constant (high-k) material such ashafnium oxide, tantalum oxide, yttrium oxide, hafnium silicate, hafniumaluminate, hafnium silicate to which nitrogen is added, or hafniumaluminate to which nitrogen is added. The use of a high-k material forthe gate insulating film 502 makes it possible to increase the thicknessin order to suppress gate leakage as well as ensuring electriccharacteristics. Note that a stacked-layer structure including a filmcontaining a high-k material and a film containing any of silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, and the likemay also be employed.

Next, in a manner similar to that of the step illustrated in FIG. 2C,the oxide semiconductor film 403 is subjected to oxygen dopingtreatment, so that an oxygen-excess region is formed (see FIG. 5C). Bythe oxygen doping treatment, the oxygen 421 is supplied to the oxidesemiconductor film 403, so that oxygen is contained in at least one ofthe interface between the base insulating film 506 and the oxidesemiconductor film 403, the inside of the oxide semiconductor film 403,and an interface between the oxide semiconductor film 403 and the gateinsulating film 502. Formation of the oxygen-excess region in the oxidesemiconductor film 403 enables an oxygen vacancy to be immediatelycompensated. Accordingly, charge trapping centers in the oxidesemiconductor film 403 can be reduced.

The oxygen doping treatment may be performed on the oxide semiconductorfilm 403 at any timing after the oxide semiconductor film 403 is formed,for example, before the source electrode layer 405 a and the drainelectrode layer 405 b are formed.

Next, a conductive film is formed over the gate insulating film 502 andsubjected to a photolithography step, so that the gate electrode layer401 is formed. After that, the insulating film 407 which covers the gateelectrode layer 401 and is in contact with the gate insulating film 502is formed (see FIG. 5D).

After the insulating film 407 is formed, heat treatment (preferablysecond heat treatment) is performed. The heat treatment is preferablyperformed at a temperature higher than or equal to 350° C. and lowerthan or equal to 650° C., further preferably higher than or equal to450° C. and lower than or equal to 650° C. or lower than the strainpoint of the substrate. The heat treatment may be performed in anatmosphere of nitrogen, oxygen, ultra-dry air (air in which the watercontent is less than or equal to 20 ppm, preferably less than or equalto 1 ppm, further preferably less than or equal to 10 ppb), or a raregas (such as argon or helium). Note that it is preferable that water,hydrogen, or the like be not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, a rare gas, or the like. The purity of nitrogen,oxygen, or a rare gas which is introduced into a heat treatmentapparatus is preferably set to be higher than or equal to 6N (99.9999%),further preferably higher than or equal to 7N (99.99999%) (that is, theimpurity concentration is lower than or equal to 1 ppm, preferably lowerthan or equal to 0.1 ppm).

Although the timing of the oxygen doping treatment and the timing of theheat treatment (second heat treatment) performed after the oxygen dopingtreatment are not limited to those in this embodiment, the heattreatment needs to be performed at least after the insulating film 407is formed. This is because, since an aluminum oxide film used as theinsulating film 407 has a high blocking effect and thus is less likelyto transmit both oxygen and an impurity such as hydrogen or moisture,release of oxygen from the oxide semiconductor film 403 can be preventedby performing the heat treatment after the insulating film 407 isformed.

Through the above steps, the transistor 510 including the oxidesemiconductor film 403 in which formation of an oxygen vacancy issuppressed is formed (see FIG. 5D). In the transistor 510, anoxygen-excess region is formed by the oxygen doping treatment, wherebyformation of an oxygen vacancy in the inside of the oxide semiconductorfilm or the interface thereof can be suppressed and the number of donorlevels in the energy gap due to oxygen vacancies can be reduced or thedonor levels can be substantially removed. Further, when oxygen issupplied to the oxide semiconductor film 403 by the oxygen dopingtreatment or the heat treatment after the oxygen doping treatment, anoxygen vacancy in the oxide semiconductor film 403 can be compensated.Moreover, a hydrogen ion remaining in the oxide semiconductor film 403can be fixed by the supplied oxygen. Therefore, fluctuation in theelectric characteristics of the transistor 510 is suppressed, and thetransistor 510 is electrically stable.

Further, the transistor 510 is preferably subjected to heat treatmentfor dehydration or dehydrogenation, and can be a transistor includingthe oxide semiconductor film 403 from which an impurity such ashydrogen, water, a hydroxyl group, or hydride (also referred to ashydrogen compound) is intentionally removed by the heat treatment. Thedehydration treatment or the dehydrogenation treatment and the oxygendoping treatment are performed on the oxide semiconductor film, wherebythe oxide semiconductor film can be highly purified so as not to containimpurities that are not main components thereof as much as possible andcan be an i-type (intrinsic) or substantially i-type oxide semiconductorfilm. The highly purified oxide semiconductor film 403 containsextremely few (close to zero) carriers.

FIGS. 6A to 6C illustrate another structural example of a transistoraccording to this embodiment. FIG. 6A is a plan view of a transistor520, and FIGS. 6B and 6C are cross-sectional views along line M-N andline O-P in FIG. 6A, respectively. Note that in FIG. 6A, some componentsof the transistor 520 (e.g., the insulating film 407) are omitted forbrevity.

As in the case of the transistor 510 illustrated in FIGS. 4A to 4C, thetransistor 520 illustrated in FIGS. 6A to 6C includes, over thesubstrate 400 having an insulating surface, the base insulating film506, the oxide semiconductor film 403, the source electrode layer 405 a,the drain electrode layer 405 b, the gate insulating film 502, the gateelectrode layer 401, and the insulating film 407.

One of differences between the transistor 520 illustrated in FIGS. 6A to6C and the transistor 510 illustrated in FIGS. 4A to 4C is the stackingorder of the source electrode layer 405 a and the drain electrode layer405 b, and the oxide semiconductor film 403. That is, the transistor 520includes the source electrode layer 405 a and the drain electrode layer405 b which are in contact with the base insulating film 506, and theoxide semiconductor film 403 which is provided over the source electrodelayer 405 a and the drain electrode layer 405 b and is at least partlyin contact with the base insulating film 506. Description of thetransistor 510 can be referred to for the details.

In the case of employing the structure illustrated in FIGS. 6A to 6C, aneffect similar to that obtained in the case of employing the structureillustrated in FIGS. 4A to 4C can be obtained.

In the transistor described in this embodiment, when the amount ofoxygen contained in the oxide semiconductor film is increased by oxygendoping treatment, degradation due to electrical bias stress or thermalstress can be suppressed and degradation due to light can be reduced.Further, an oxygen-excess region is formed in the oxide semiconductorfilm by the oxygen doping treatment, whereby an oxygen vacancy in thefilm can be compensated. Furthermore, an impurity containing a hydrogenatom such as hydrogen, water, a hydroxyl group, or hydride (alsoreferred to as hydrogen compound) is removed from the oxidesemiconductor by heat treatment, whereby the oxide semiconductor filmcan be highly purified and become i-type (intrinsic). Inclusion of suchan oxide semiconductor film enables the transistor to have lessfluctuation in electric characteristics such as threshold voltage and tobe electrically stable.

As described above, according to this embodiment, a semiconductor devicewhich is formed using an oxide semiconductor and has stable electriccharacteristics can be provided. Further, a semiconductor device havinghigh reliability can be provided.

The methods, structures, and the like described in this embodiment canbe combined as appropriate with any of the methods, structures, and thelike described in the other embodiments.

Embodiment 3

A semiconductor device having a display function (also referred to asdisplay device) can be manufactured using the transistor described inEmbodiment 1 or 2. Moreover, some or all of driver circuits whichinclude transistors can be formed over a substrate where a pixel portionis formed, whereby a system-on-panel can be obtained.

In FIG. 8A, a pixel portion 4002 provided over a first substrate 4001 issealed by a second substrate 4006 and a sealant 4005 provided so as tosurround the pixel portion 4002. In FIG. 8A, a signal line drivercircuit 4003 and a scan line driver circuit 4004 which are each formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate prepared separately are mounted inregions that are different from the region surrounded by the sealant4005 over the first substrate 4001. Various signals and potentials aresupplied to the signal line driver circuit 4003, the scan line drivercircuit 4004, and the pixel portion 4002 from flexible printed circuits(FPCs) 4018 a and 4018 b.

In FIGS. 8B and 8C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Consequently, the pixel portion 4002 including a display elementand the scan line driver circuit 4004 are sealed together by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. InFIGS. 8B and 8C, the signal line driver circuit 4003 which is formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate prepared separately is mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. In FIGS. 8B and 8C, various signals andpotentials are supplied to the signal line driver circuit 4003 which isseparately formed, the scan line driver circuit 4004, and the pixelportion 4002 from an FPC 4018.

Although FIGS. 8B and 8C each illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, one embodiment of the present invention is not limitedto this structure. The scan line driver circuit may be formed separatelyand then mounted, or only part of the signal line driver circuit or partof the scan line driver circuit may be formed separately and thenmounted.

Note that there is no particular limitation on the method of connectinga separately formed driver circuit, and a chip on glass (COG) method, awire bonding method, a tape automated bonding (TAB) method, or the likecan be used. FIG. 8A illustrates an example in which the signal linedriver circuit 4003 and the scan line driver circuit 4004 are mounted bya COG method. FIG. 8B illustrates an example in which the signal linedriver circuit 4003 is mounted by a COG method. FIG. 8C illustrates anexample in which the signal line driver circuit 4003 is mounted by a TABmethod.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Furthermore, the display device also includes the followingmodules in its category: a module to which a connector such as an FPC, aTAB tape, or a TCP is attached; a module having a TAB tape or a TCP atthe tip of which a printed wiring board is provided; and a module inwhich an integrated circuit (IC) is directly mounted on a displayelement by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors, and the transistordescribed in Embodiment 1 or 2 can be applied thereto.

As the display element provided in the display device, a liquid crystalelement (also referred to as liquid crystal display element) or alight-emitting element (also referred to as light-emitting displayelement) can be used. The light-emitting element includes, in itscategory, an element whose luminance is controlled by current or voltageand specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Furthermore, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

Embodiments of the semiconductor device will be described with referenceto FIG. 9, FIG. 10, and FIG. 11. FIG. 9, FIG. 10, and FIG. 11 correspondto cross-sectional views along line Q-R in FIG. 8B.

As illustrated in FIG. 9, FIG. 10, and FIG. 11, the semiconductor deviceincludes a connection terminal electrode layer 4015 and a terminalelectrode layer 4016. The connection terminal electrode layer 4015 andthe terminal electrode layer 4016 are electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

The connection terminal electrode layer 4015 is formed using the sameconductive film as a first electrode layer 4030, and the terminalelectrode layer 4016 is formed using the same conductive film as sourceelectrode layers and drain electrode layers of transistors 4010 and4011.

The pixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001 include a plurality oftransistors. In FIG. 9, FIG. 10, and FIG. 11, the transistor 4010included in the pixel portion 4002 and the transistor 4011 included inthe scan line driver circuit 4004 are illustrated as an example. In FIG.9, an insulating film 4020 and an insulating film 4024 are provided overthe transistors 4010 and 4011. In FIG. 10 and FIG. 11, an insulatingfilm 4021 is further provided. Note that an insulating film 4023 is aninsulating film functioning as a base film.

In this embodiment, the transistor described in Embodiment 1 or 2 can beapplied to the transistor 4010 and the transistor 4011.

The transistor 4010 and the transistor 4011 are each a transistorincluding an oxide semiconductor film which is highly purified and inwhich formation of an oxygen vacancy is suppressed. Therefore,fluctuation in the electric characteristics of the transistor 4010 andthe transistor 4011 is suppressed, and the transistor 4010 and thetransistor 4011 are electrically stable.

Accordingly, highly reliable semiconductor devices can be provided asthe semiconductor devices of this embodiment in FIG. 9, FIG. 10, andFIG. 11.

In addition, an example is described in this embodiment, in which aconductive layer is provided over the insulating film so as to overlapwith a channel formation region of the oxide semiconductor film of thetransistor 4011 for the driver circuit. By providing the conductivelayer so as to overlap with the channel formation region of the oxidesemiconductor film, the amount of change in the threshold voltage of thetransistor 4011 by a BT test can be further reduced. The conductivelayer may have the same potential as or a potential different from thatof a gate electrode layer of the transistor 4011, and can function as asecond gate electrode layer. The potential of the conductive layer maybe GND or 0 V, or the conductive layer may be in a floating state.

In addition, the conductive layer functions to block an externalelectric field, that is, to prevent an external electric field(particularly, to prevent static electricity) from affecting the inside(a circuit portion including a thin film transistor). A blockingfunction of the conductive layer can prevent fluctuation in the electriccharacteristics of the transistor due to an influence of an externalelectric field such as static electricity.

The transistor 4010 provided in the pixel portion 4002 is electricallyconnected to a display element to form a display panel. A variety ofdisplay elements can be used as the display element as long as displaycan be performed.

An example of a liquid crystal display device using a liquid crystalelement as a display element is illustrated in FIG. 9. In FIG. 9, aliquid crystal element 4013 which is a display element includes thefirst electrode layer 4030, a second electrode layer 4031, and a liquidcrystal layer 4008. Insulating films 4032 and 4033 functioning asalignment films are provided so that the liquid crystal layer 4008 isinterposed therebetween. The second electrode layer 4031 is provided onthe second substrate 4006 side, and the first electrode layer 4030 andthe second electrode layer 4031 are stacked with the liquid crystallayer 4008 interposed therebetween.

A columnar spacer denoted by reference numeral 4035 is obtained byselective etching of an insulating film and is provided in order tocontrol the thickness (cell gap) of the liquid crystal layer 4008.Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while the temperature of a cholestericliquid crystal is increased. Since the blue phase appears only in anarrow temperature range, a liquid crystal composition in which severalweight percent or more of a chiral agent is mixed is used for the liquidcrystal layer in order to improve the temperature range. The liquidcrystal composition which includes a liquid crystal exhibiting a bluephase and a chiral agent has a short response time, and has opticalisotropy, which makes the alignment process unneeded and viewing angledependence small. In addition, since an alignment film does not need tobe provided and rubbing treatment is unnecessary, electrostaticdischarge damage caused by the rubbing treatment can be prevented anddefects and damage of the liquid crystal display device in themanufacturing process can be reduced. Thus, productivity of the liquidcrystal display device can be improved. A transistor formed using anoxide semiconductor film has a possibility that the electriccharacteristics of the transistor may fluctuate significantly by theinfluence of static electricity and deviate from the designed range.Therefore, it is more effective to use a liquid crystal materialexhibiting a blue phase for the liquid crystal display device includingthe transistor formed using an oxide semiconductor film.

The specific resistivity of the liquid crystal material is higher thanor equal to 1×10⁹ Ω·cm, preferably higher than or equal to 1×10¹¹ Ω·cm,further preferably higher than or equal to 1×10¹² Ω·cm. Note that thespecific resistivity in this specification is measured at 20° C.

The size of a storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. The size of the storage capacitor may be setconsidering the off-state current of the transistor or the like. Byusing a transistor including an oxide semiconductor film which is highlypurified and includes an oxygen-excess region, it is enough to provide astorage capacitor having a capacitance that is ⅓ or less, preferably ⅕or less of liquid crystal capacitance of each pixel.

In the transistor used in this embodiment, which includes an oxidesemiconductor film which is highly purified and in which formation of anoxygen vacancy is suppressed, the current in an off state (off-statecurrent) can be made small. Accordingly, an electric signal such as animage signal can be held for a longer period, and a writing interval canbe set longer in an on state. Accordingly, frequency of refreshoperation can be reduced, which leads to an effect of suppressing powerconsumption.

The transistor used in this embodiment, which includes an oxidesemiconductor film which is highly purified and in which formation of anoxygen vacancy is suppressed, can have relatively high field-effectmobility and thus can operate at high speed. For example, when such atransistor which can operate at high speed is used for a liquid crystaldisplay device, a switching transistor in a pixel portion and a drivertransistor in a driver circuit portion can be formed over one substrate.That is, since a semiconductor device formed of a silicon wafer or thelike is not additionally needed as a driver circuit, the number ofcomponents of the semiconductor device can be reduced. In addition, byusing a transistor which can operate at high speed in a pixel portion, ahigh-quality image can be provided.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modemay be used. Some examples are given as the vertical alignment mode. Forexample, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, or an ASV mode can be used. Furthermore,this embodiment can be applied to a VA liquid crystal display device.The VA liquid crystal display device has a kind of form in whichalignment of liquid crystal molecules of a liquid crystal display panelis controlled. In the VA liquid crystal display device, liquid crystalmolecules are aligned in a vertical direction with respect to a panelsurface when no voltage is applied. Moreover, it is possible to use amethod called domain multiplication or multi-domain design, in which apixel is divided into some regions (subpixels) and molecules are alignedin different directions in their respective regions.

In the display device, a black matrix (light-blocking layer), an opticalmember (optical substrate) such as a polarizing member, a retardationmember, or an anti-reflection member, and the like are provided asappropriate. For example, circular polarization may be obtained by usinga polarizing substrate and a retardation substrate. In addition, abacklight, a side light, or the like may be used as a light source.

As a display method in the pixel portion, a progressive method, aninterlace method, or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite); R, G, B, and one or more of yellow, cyan, magenta, and the like;or the like can be used. Further, the sizes of display regions may bedifferent between respective dots of color elements. Note that oneembodiment of the invention disclosed herein is not limited to theapplication to a display device for color display; one embodiment of theinvention disclosed herein can also be applied to a display device formonochrome display.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as organicEL element, and the latter is referred to as inorganic EL element.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as current-excitationlight-emitting element.

Inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is interposed between dielectriclayers, which are further interposed between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element is described here as a light-emitting element.

In order to extract light emitted from the light-emitting element, atleast one of the pair of electrodes has a light-transmitting property. Atransistor and a light-emitting element are formed over a substrate. Thelight-emitting element can have a top emission structure in which lightemission is extracted through a surface opposite to the substrate; abottom emission structure in which light emission is extracted through asurface on the substrate side; or a dual emission structure in whichlight emission is extracted through the surface opposite to thesubstrate and the surface on the substrate side, and a light-emittingelement having any of these emission structures can be used.

An example of a light-emitting device in which a light-emitting elementis used as a display element is illustrated in FIG. 10. A light-emittingelement 4513 which is a display element is electrically connected to thetransistor 4010 provided in the pixel portion 4002. A structure of thelight-emitting element 4513 is not limited to the stacked-layerstructure including the first electrode layer 4030, anelectroluminescent layer 4511, and the second electrode layer 4031,which is illustrated in FIG. 10. The structure of the light-emittingelement 4513 can be changed as appropriate depending on the direction inwhich light is extracted from the light-emitting element 4513, or thelike.

A partition wall 4510 can be formed using an organic insulating materialor an inorganic insulating material. It is particularly preferable thatthe partition wall 4510 be formed using a photosensitive resin materialto have an opening over the first electrode layer 4030 so that asidewall of the opening is formed as a tilted surface with continuouscurvature.

The electroluminescent layer 4511 may be formed using either a singlelayer or a plurality of layers stacked.

A protective film may be formed over the second electrode layer 4031 andthe partition wall 4510 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4513. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed. In addition, in aspace which is formed with the first substrate 4001, the secondsubstrate 4006, and the sealant 4005, a filler 4514 is provided forsealing. In this manner, the light-emitting element 4513 and the likeare preferably packaged (sealed) with a protective film (such as alaminate film or an ultraviolet curable resin film) or a cover materialwith high air-tightness and little degasification so that thelight-emitting element 4513 and the like are not exposed to the outsideair.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon. Forexample, polyvinyl chloride (PVC), an acrylic resin, polyimide, an epoxyresin, a silicone resin, polyvinyl butyral (PVB), or ethylene vinylacetate (EVA) can be used. For example, nitrogen is used as the filler.

In addition, as needed, an optical film such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Further, electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also referred toas electrophoretic display device (electrophoretic display) and isadvantageous in that it has the same level of readability as plainpaper, it has lower power consumption than other display devices, and itcan be made thin and lightweight.

An electrophoretic display device can have various modes. Anelectrophoretic display device contains a plurality of microcapsulesdispersed in a solvent or a solute, and each microcapsule contains firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the particles in the microcapsules move in opposite directions to eachother and only the color of the particles gathering on one side isdisplayed. Note that the first particles and the second particles eachcontain a pigment and do not move without an electric field. Moreover,the first particles and the second particles have different colors(which may be colorless).

Thus, an electrophoretic display device is a display that utilizes aso-called dielectrophoretic effect by which a substance having a highdielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material of any ofthese.

As the electronic paper, a display device using a twisting ball displaysystem can be used. The twisting ball display system refers to a methodin which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control the orientation of the spherical particles,so that display is performed.

FIG. 11 illustrates active matrix electronic paper as one embodiment ofa semiconductor device. The electronic paper in FIG. 11 is an example ofa display device using a twisting ball display system. The twisting balldisplay system refers to a method in which spherical particles eachcolored in black and white are arranged between electrode layers usedfor a display element, and a potential difference is generated betweenthe electrode layers to control the orientation of the sphericalparticles, so that display is performed.

Between the first electrode layer 4030 connected to the transistor 4010and the second electrode layer 4031 provided with the second substrate4006, spherical particles 4613 each of which includes a sphericalparticle having a black region 4615 a and a white region 4615 b includedin a cavity 4612 which is filled with liquid are provided. A spacearound the spherical particles 4613 is filled with a filler 4614 such asa resin. The second electrode layer 4031 corresponds to a commonelectrode layer (counter electrode layer). The second electrode layer4031 is electrically connected to a common potential line.

Note that in FIG. 9, FIG. 10, and FIG. 11, a flexible substrate as wellas a glass substrate can be used as the first substrate 4001 and thesecond substrate 4006. For example, a plastic substrate having alight-transmitting property or the like can be used. As plastic, afiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF)film, a polyester film, or an acrylic resin film can be used. Inaddition, a sheet with a structure in which an aluminum foil isinterposed between PVF films or polyester films can be used.

In this embodiment, a silicon oxide film is used as the insulating film4020, and an aluminum oxide film is used as the insulating film 4024.The insulating film 4020 and the insulating film 4024 can be formed by asputtering method or a plasma CVD method.

The aluminum oxide film provided as the insulating film 4024 over theoxide semiconductor film has a high blocking effect and thus is lesslikely to transmit both oxygen and an impurity such as hydrogen ormoisture.

Therefore, during the manufacturing process and after the manufacture,the aluminum oxide film functions as a protective film for preventingentry of an impurity such as hydrogen or moisture, which can cause achange, into the oxide semiconductor film and release of oxygen, whichis a main component material of the oxide semiconductor, from the oxidesemiconductor film.

The silicon oxide film provided as the insulating film 4020 in contactwith the oxide semiconductor film has a function of supplying oxygen tothe oxide semiconductor film. Therefore, the insulating film 4020 ispreferably an oxide insulating film containing much oxygen.

The transistor 4010 and the transistor 4011 each include an oxidesemiconductor film which is highly purified and in which formation of anoxygen vacancy is suppressed. In addition, the transistor 4010 and thetransistor 4011 each include a silicon oxide film as a gate insulatingfilm. The oxide semiconductor film included in each of the transistor4010 and the transistor 4011 is obtained in such a manner that a regioncontaining more oxygen than the stoichiometric composition is formed byoxygen doping treatment, and heat treatment is performed after thedoping in a state where the aluminum oxide film is provided as theinsulating film 4024 over the oxide semiconductor film; therefore,oxygen can be prevented from being released from the oxide semiconductorfilm by the heat treatment. Accordingly, the obtained oxidesemiconductor film can be a film which includes a region containing moreoxygen than the stoichiometric composition.

The oxide semiconductor film included in each of the transistor 4010 andthe transistor 4011 is a highly purified film which is dehydrated ordehydrogenated by at least one of heat treatment performed afterformation of the oxide semiconductor film and the heat treatmentperformed after the oxygen doping treatment. Accordingly, by using theoxide semiconductor film for each of the transistor 4010 and thetransistor 4011, fluctuation in the threshold voltage Vth of thetransistors due to an oxygen vacancy and a shift of the thresholdvoltage ΔVth can be reduced.

Further, the insulating film 4021 functioning as a planarizationinsulating film can be formed using an organic material having heatresistance, such as an acrylic resin, polyimide, abenzocyclobutene-based resin, polyamide, or an epoxy resin. Other thansuch organic materials, it is also possible to use a low-dielectricconstant material (low-k material), a siloxane-based resin,phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or thelike. Alternatively, the insulating film 4021 may be formed by stackinga plurality of insulating films formed using any of these materials.

There is no particular limitation on the method of forming theinsulating film 4021, and the following method or tool (equipment) canbe used depending on the material: a sputtering method, an SOG method,spin coating, dipping, spray coating, a droplet discharge method (suchas an inkjet method), screen printing, offset printing, a doctor knife,a roll coater, a curtain coater, a knife coater, or the like.

The display device displays an image by transmitting light from a lightsource or a display element. Therefore, the substrate and the thin filmssuch as the insulating film and the conductive film provided for thepixel portion where light is transmitted have light-transmittingproperties with respect to light in the visible light wavelength range.

The first electrode layer and the second electrode layer (each of whichmay be called pixel electrode layer, common electrode layer, counterelectrode layer, or the like) for applying voltage to the displayelement may have light-transmitting properties or light-reflectingproperties, which depends on the direction in which light is extracted,the position where the electrode layer is provided, the patternstructure of the electrode layer, and the like.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (referred to as ITO), indiumzinc oxide, indium tin oxide to which silicon oxide is added, orgraphene.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed using one or plural kinds selected from a metal such astungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), orsilver (Ag); an alloy thereof; and a nitride thereof.

A conductive composition containing a conductive high molecule (alsoreferred to as conductive polymer) can be used for the first electrodelayer 4030 and the second electrode layer 4031. As the conductive highmolecule, a so-called π-electron conjugated conductive polymer can beused. For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, polythiophene or a derivative thereof, a copolymerof two or more of aniline, pyrrole, and thiophene or a derivativethereof can be given.

Since the transistor is easily broken owing to static electricity or thelike, a protection circuit for protecting the driver circuit ispreferably provided. The protection circuit is preferably formed using anonlinear element.

By using the transistor described in Embodiment 1 or 2 as describedabove, the semiconductor device can have a variety of functions.

Embodiment 4

A semiconductor device having an image sensor function of readinginformation on an object can be formed with the use of the transistordescribed in Embodiment 1 or 2.

An example of a semiconductor device having an image sensor function isillustrated in FIG. 12A. FIG. 12A illustrates an equivalent circuit of aphoto sensor, and FIG. 12B is a cross-sectional view illustrating partof the photo sensor.

One electrode of a photodiode 602 is electrically connected to aphotodiode reset signal line 658, and the other electrode of thephotodiode 602 is electrically connected to a gate of a transistor 640.One of a source and a drain of the transistor 640 is electricallyconnected to a photo sensor reference signal line 672, and the other ofthe source and the drain of the transistor 640 is electrically connectedto one of a source and a drain of a transistor 656. A gate of thetransistor 656 is electrically connected to a gate signal line 659, andthe other of the source and the drain of the transistor 656 iselectrically connected to a photo sensor output signal line 671.

Note that in circuit diagrams in this specification, a transistor formedusing an oxide semiconductor film is denoted by a symbol “OS” so that itcan be identified as a transistor formed using an oxide semiconductorfilm. In FIG. 12A, the transistor 640 and the transistor 656 are each atransistor formed using an oxide semiconductor film in which anoxygen-excess region is formed by oxygen doping treatment, like thetransistor described in Embodiment 1 or 2.

FIG. 12B is a cross-sectional view of the photodiode 602 and thetransistor 640 in the photo sensor. The photodiode 602 functioning as asensor and the transistor 640 are provided over a substrate 601 (TFTsubstrate) having an insulating surface. A substrate 613 is providedover the photodiode 602 and the transistor 640 with the use of anadhesive layer 608.

An insulating film 631, an insulating film 632, an interlayer insulatingfilm 633, and an interlayer insulating film 634 are provided over thetransistor 640. The photodiode 602 is provided over the interlayerinsulating film 633. In the photodiode 602, a first semiconductor film606 a, a second semiconductor film 606 b, and a third semiconductor film606 c are sequentially stacked from the interlayer insulating film 633side, between an electrode layer 641 a formed over the interlayerinsulating film 633 and an electrode layer 642 formed over theinterlayer insulating film 634.

The electrode layer 641 a is electrically connected to a conductivelayer 643 formed over the interlayer insulating film 634, and theelectrode layer 642 is electrically connected to a gate electrode layer645 through the electrode layer 641 b. The gate electrode layer 645 iselectrically connected to a gate electrode layer of the transistor 640,and the photodiode 602 is electrically connected to the transistor 640.

Here, a pin photodiode in which a semiconductor film having p-typeconductivity type as the first semiconductor film 606 a, ahigh-resistance semiconductor film (i-type semiconductor film) as thesecond semiconductor film 606 b, and a semiconductor film having n-typeconductivity type as the third semiconductor film 606 c are stacked isillustrated as an example.

The first semiconductor film 606 a is a p-type semiconductor film andcan be formed using an amorphous silicon film containing an impurityelement imparting p-type conductivity type. The first semiconductor film606 a is formed by a plasma CVD method with the use of a semiconductorsource gas containing an impurity element belonging to Group 13 (e.g.,boron (B)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then an impurity elementmay be introduced into the amorphous silicon film by a diffusion methodor an ion implantation method. Heating or the like may be conductedafter introducing the impurity element by an ion implantation method orthe like in order to diffuse the impurity element. In this case, as amethod of forming the amorphous silicon film, an LPCVD method, a vapordeposition method, a sputtering method, or the like may be used. Thefirst semiconductor film 606 a is preferably formed to have a thicknessgreater than or equal to 10 nm and less than or equal to 50 nm.

The second semiconductor film 606 b is an i-type semiconductor film(intrinsic semiconductor film) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor film 606 b, anamorphous silicon film is formed by a plasma CVD method with the use ofa semiconductor source gas. As the semiconductor source gas, silane(SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄,or the like may be used. The second semiconductor film 606 b may beformed by an LPCVD method, a vapor deposition method, a sputteringmethod, or the like. The second semiconductor film 606 b is preferablyformed to have a thickness greater than or equal to 200 nm and less thanor equal to 1000 nm.

The third semiconductor film 606 c is an n-type semiconductor film andis formed using an amorphous silicon film containing an impurity elementimparting n-type conductivity type. The third semiconductor film 606 cis formed by a plasma CVD method with the use of a semiconductor sourcegas containing an impurity element belonging to Group 15 (e.g.,phosphorus (P)). As the semiconductor source gas, silane (SiH₄) may beused. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the likemay be used. Further alternatively, an amorphous silicon film which doesnot contain an impurity element may be formed, and then an impurityelement may be introduced into the amorphous silicon film by a diffusionmethod or an ion implantation method. Heating or the like may beconducted after introducing the impurity element by an ion implantationmethod or the like in order to diffuse the impurity element. In thiscase, as a method of forming the amorphous silicon film, an LPCVDmethod, a vapor deposition method, a sputtering method, or the like maybe used. The third semiconductor film 606 c is preferably formed to havea thickness greater than or equal to 20 nm and less than or equal to 200nm.

The first semiconductor film 606 a, the second semiconductor film 606 b,and the third semiconductor film 606 c are not necessarily formed usingan amorphous semiconductor, and may be formed using a polycrystallinesemiconductor or a microcrystalline semiconductor (semi-amorphoussemiconductor: SAS).

The microcrystalline semiconductor belongs to a metastable state of anintermediate between amorphous and single crystalline when Gibbs freeenergy is considered. That is, the microcrystalline semiconductor is asemiconductor having a third state which is stable in terms of freeenergy and has a short range order and lattice distortion. Columnar-likeor needle-like crystals grow in a normal direction with respect to asubstrate surface. The Raman spectrum of microcrystalline silicon, whichis a typical example of a microcrystalline semiconductor, is located inlower wave numbers than 520 cm⁻¹, which represents a peak of the Ramanspectrum of single crystal silicon. That is, the peak of the Ramanspectrum of microcrystalline silicon exists between 520 cm⁻¹ whichrepresents single crystal silicon and 480 cm⁻¹ which representsamorphous silicon. In addition, microcrystalline silicon contains atleast 1 at. % of hydrogen or halogen in order to terminate a danglingbond. Moreover, microcrystalline silicon contains a rare gas elementsuch as helium, argon, krypton, or neon to further promote latticedistortion, so that stability is increased and a favorablemicrocrystalline semiconductor film can be obtained.

This microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens ofmegahertz to several hundreds of megahertz or using a microwave plasmaCVD apparatus with a frequency of 1 GHz or higher. The microcrystallinesemiconductor film can be typically formed using a dilution of siliconhydride such as SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, or SiF₄ withhydrogen. With a dilution with one or a plural kinds of rare gaselements selected from helium, argon, krypton, and neon in addition tosilicon hydride and hydrogen, the microcrystalline semiconductor filmcan be formed. In that case, the flow ratio of hydrogen to siliconhydride is 5:1 to 200:1, preferably 50:1 to 150:1, further preferably100:1. Further, a carbide gas such as CH₄ or C₂H₆, a germanium gas suchas GeH₄ or GeF₄, F₂, or the like may be mixed into the gas containingsilicon.

In addition, the mobility of holes generated by a photoelectric effectis lower than the mobility of electrons. Therefore, a pin photodiode hasbetter characteristics when a surface on the p-type semiconductor filmside is used as a light-receiving plane. Here, an example in which light622 received by the photodiode 602 from a surface of the substrate 601,over which the pin photodiode is formed, is converted into electricsignals is described. Further, light from the semiconductor film havinga conductivity type opposite to that of the semiconductor film on thelight-receiving plane is disturbance light; therefore, the electrodelayer is preferably formed using a light-blocking conductive film. Notethat a surface on the n-type semiconductor film side can alternativelybe used as the light-receiving plane.

With the use of an insulating material, the insulating film 632, theinterlayer insulating film 633, and the interlayer insulating film 634can be formed, depending on the material, using a method or a tool(equipment) such as a sputtering method, a plasma CVD method, an SOGmethod, spin coating, dipping, spray coating, a droplet discharge method(such as an inkjet method), screen printing, or offset printing, adoctor knife, a roll coater, a curtain coater, or a knife coater.

In this embodiment, an aluminum oxide film is used as the insulatingfilm 631. The insulating film 631 can be formed by a sputtering methodor a plasma CVD method.

The aluminum oxide film provided as the insulating film 631 over theoxide semiconductor film has a high blocking effect and thus is lesslikely to transmit both oxygen and an impurity such as hydrogen ormoisture.

Therefore, during the manufacturing process and after the manufacture,the aluminum oxide film functions as a protective film for preventingentry of an impurity such as hydrogen or moisture, which can cause achange, into the oxide semiconductor film and release of oxygen, whichis a main component material of the oxide semiconductor, from the oxidesemiconductor film.

In this embodiment, the transistor 640 includes an oxide semiconductorfilm which is highly purified and in which formation of an oxygenvacancy is suppressed. In addition, the transistor 640 includes asilicon oxide film as a gate insulating film. The oxide semiconductorfilm included in the transistor 640 is obtained in such a manner that aregion containing more oxygen than the stoichiometric composition isformed by oxygen doping treatment, and heat treatment is performed afterthe doping in a state where the aluminum oxide film is provided as theinsulating film 631 over the oxide semiconductor film; therefore, oxygencan be prevented from being released from the oxide semiconductor filmby the heat treatment. Accordingly, the obtained oxide semiconductorfilm can be a film which includes a region containing more oxygen thanthe stoichiometric composition.

The oxide semiconductor film included in the transistor 640 is a highlypurified film which is dehydrated or dehydrogenated by at least one ofheat treatment performed after formation of the oxide semiconductor filmand the heat treatment performed after the oxygen doping treatment.Accordingly, by using the oxide semiconductor film for the transistor640, fluctuation in the threshold voltage Vth of the transistor due toan oxygen vacancy and a shift of the threshold voltage ΔVth can bereduced.

The insulating film 632 can be formed using an inorganic insulatingmaterial and can have a single-layer structure or a stacked-layerstructure including any of oxide insulating films such as a siliconoxide layer, a silicon oxynitride layer, an aluminum oxide layer, and analuminum oxynitride layer; and nitride insulating films such as asilicon nitride layer, a silicon nitride oxide layer, an aluminumnitride layer, and an aluminum nitride oxide layer.

For a reduction in surface roughness, an insulating film functioning asa planarization insulating film is preferably used as each of theinterlayer insulating films 633 and 634. For the interlayer insulatingfilms 633 and 634, for example, an organic insulating material havingheat resistance such as polyimide, an acrylic resin, a benzocyclobuteneresin, polyamide, or an epoxy resin can be used. Other than such organicinsulating materials, it is possible to use a single layer or stackedlayers of a low-dielectric constant material (low-k material), asiloxane-based resin, phosphosilicate glass (PSG), borophosphosilicateglass (BPSG), or the like.

With detection of light that enters the photodiode 602, information onan object to be detected can be read. Note that a light source such as abacklight can be used at the time of reading information on the objectto be detected.

As described above, fluctuation in the electric characteristics of atransistor including an oxide semiconductor film which is highlypurified and contains excess oxygen that compensates an oxygen vacancyis suppressed, and the transistor is electrically stable. Thus, by usingthe transistor, a highly reliable semiconductor device can be provided.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 5

The transistor described in Embodiment 1 or 2 can be favorably used fora semiconductor device including an integrated circuit in which aplurality of transistors is stacked. In this embodiment, as an exampleof the semiconductor device, a memory medium (memory element) will bedescribed.

In this embodiment, a semiconductor device including a first transistorwhich is formed using a single crystal semiconductor substrate and asecond transistor which is formed using a semiconductor film and isprovided above the first transistor with an insulating film positionedbetween the first transistor and the second transistor is manufactured.

FIGS. 7A to 7C illustrate an example of a structure of a semiconductordevice. FIG. 7A illustrates a cross section of the semiconductor device,and FIG. 7B illustrates a plan view of the semiconductor device. Here,FIG. 7A corresponds to a cross section along line C1-C2 line D1-D2 inFIG. 7B. FIG. 7C is an example of a diagram of a circuit using thesemiconductor device as a memory element.

The semiconductor device illustrated in FIGS. 7A and 7B includes atransistor 140 formed using a first semiconductor material in a lowerportion, and a transistor 162 formed using a second semiconductormaterial in an upper portion. The transistor described in Embodiment 1or 2 can be favorably used as the transistor 162. In this embodiment, anexample in which a transistor having a structure similar to that of thetransistor 510 described in Embodiment 2 is used as the transistor 162will be described.

Semiconductor materials and structures of the transistor 140 and thetransistor 162, which are stacked, may be the same or different. In thisembodiment, an example is described in which materials and structureswhich are appropriate for the circuit of the memory medium (memoryelement) are employed for the transistors; here, the first semiconductormaterial is a semiconductor material other than an oxide semiconductor,and the second semiconductor material is an oxide semiconductor. As thesemiconductor material other than an oxide semiconductor, for example,silicon, germanium, silicon germanium, silicon carbide, or galliumarsenide can be used, and a single crystal semiconductor is preferablyused. Alternatively, an organic semiconductor material or the like maybe used. A transistor formed using such a semiconductor material canoperate at high speed easily. On the other hand, a transistor formedusing an oxide semiconductor can hold charge for a long time owing toits characteristics.

The transistor 140 includes a channel formation region 116 provided in asubstrate 185 containing a semiconductor material (e.g., silicon),impurity regions 120 provided so that the channel formation region 116is interposed therebetween, metal compound regions 124 in contact withthe impurity regions 120, a gate insulating film 108 provided over thechannel formation region 116, and a gate electrode layer 110 providedover the gate insulating film 108.

As the substrate 185 containing a semiconductor material, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate of silicon, silicon carbide, or the like; a compoundsemiconductor substrate of silicon germanium or the like; an SOIsubstrate; or the like can be used. Note that although the term “SOIsubstrate” generally means a substrate in which a silicon semiconductorfilm is provided over an insulating surface, the term “SOI substrate” inthis specification and the like also includes a substrate in which asemiconductor film formed using a material other than silicon isprovided over an insulating surface. That is, a semiconductor filmincluded in the “SOI substrate” is not limited to a siliconsemiconductor film. Moreover, the SOI substrate can be a substratehaving a structure where a semiconductor film is provided over aninsulating substrate such as a glass substrate with an insulating filmpositioned therebetween.

As a method of forming the SOI substrate, any of the following methodscan be used: a method in which oxygen ions are implanted into aminor-polished wafer and then heating is performed at a hightemperature, whereby an oxide layer is formed at a certain depth from asurface of the wafer and a defect caused in the surface layer iseliminated; a method in which a semiconductor substrate is separated byutilizing a phenomenon in which microvoids formed by hydrogen ionirradiation grow because of heat treatment; a method in which a singlecrystal semiconductor film is formed over an insulating surface bycrystal growth; and the like.

For example, ions are added through one surface of a single crystalsemiconductor substrate, an embrittlement layer is formed at a certaindepth from the surface of the single crystal semiconductor substrate,and an insulating film is formed over one of the surface of the singlecrystal semiconductor substrate and an element substrate. Heat treatmentis performed in a state where the single crystal semiconductor substrateand the element substrate are bonded to each other with the insulatingfilm interposed therebetween, so that a crack is generated in theembrittlement layer and the single crystal semiconductor substrate isseparated along the embrittlement layer. Accordingly, a single crystalsemiconductor film, which is separated from the single crystalsemiconductor substrate, is formed as a semiconductor film over theelement substrate. An SOI substrate formed by the above method can alsobe favorably used.

An element isolation insulating film 106 is provided on the substrate185 so as to surround the transistor 140. Note that for highintegration, it is preferable that, as in FIG. 7A, the transistor 140 donot include a sidewall insulating film. On the other hand, in the casewhere the characteristics of the transistor 140 have priority, asidewall insulating film may be provided on a side surface of the gateelectrode layer 110, and the impurity region 120 may include a regionhaving a different impurity concentration.

The transistor 140 formed using a single crystal semiconductor substratecan operate at high speed. Thus, when the transistor is used as areading transistor, data can be read at high speed.

In this embodiment, two insulating films are formed so as to cover thetransistor 140. Note that an insulating film which covers the transistor140 may also have a single-layer structure or a stacked-layer structureof three or more layers. Note also that a silicon oxide film is used asan insulating film in contact with an oxide semiconductor film includedin the transistor 162 which is provided in the upper portion.

As treatment prior to formation of the transistor 162 and a capacitor164, CMP treatment is performed on the two insulating films, whereby aninsulating film 128 and an insulating film 130 which are planarized areformed and, at the same time, an upper surface of the gate electrodelayer 110 is exposed.

As each of the insulating film 128 and the insulating film 130,typically, it is possible to use an inorganic insulating film such as asilicon oxide film, a silicon oxynitride film, an aluminum oxide film,an aluminum oxynitride film, a silicon nitride film, an aluminum nitridefilm, a silicon nitride oxide film, or an aluminum nitride oxide film.The insulating film 128 and the insulating film 130 can be formed by aplasma CVD method, a sputtering method, or the like.

Alternatively, an organic material such as polyimide, an acrylic resin,or a benzocyclobutene resin can be used. Other than such organicmaterials, it is also possible to use a low-dielectric constant material(low-k material) or the like. In the case of using an organic material,the insulating film 128 and the insulating film 130 may be formed by awet method such as a spin coating method or a printing method.

In this embodiment, a 50-nm-thick silicon oxynitride film is formed asthe insulating film 128 by a sputtering method, and a 550-nm-thicksilicon oxide film is formed as the insulating film 130 by a sputteringmethod.

After that, an oxide semiconductor film is formed over the insulatingfilm 130 which is sufficiently planarized by the CMP treatment and isprocessed, so that an island-shaped oxide semiconductor film 144 isformed. Note that after the formation of the oxide semiconductor film,heat treatment for dehydration or dehydrogenation is preferablyperformed.

Next, a conductive layer is formed over the gate electrode layer 110,the insulating film 128, the insulating film 130, and the like andetched selectively, so that a source or drain electrode layer 142 a anda drain or source electrode layer 142 b are formed.

The conductive layer can be formed by a PVD method such as a sputteringmethod or a CVD method such as a plasma CVD method. Further, as amaterial of the conductive layer, an element selected from Al, Cr, Cu,Ta, Ti, Mo, and W, an alloy containing any of the above elements as acomponent, or the like can be used. Any of Mn, Mg, Zr, Be, Nd, and Sc,or a material containing any of these in combination may be used.

The conductive layer may have a single-layer structure or astacked-layer structure of two or more layers. For example, theconductive layer can have a single-layer structure of a titanium film ora titanium nitride film, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, or a three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order. Note that the conductive layer having asingle-layer structure of a titanium film or a titanium nitride film hasan advantage in that it can be easily processed into the source or drainelectrode layer 142 a and the drain or source electrode layer 142 bhaving a tapered shape.

The channel length (L) of the transistor 162 in the upper portion isdetermined by the distance between a lower edge portion of the source ordrain electrode layer 142 a and a lower edge portion of the drain orsource electrode layer 142 b. Note that for light exposure for forming amask used in the case where a transistor with a channel length (L) lessthan 25 nm is formed, it is preferable to use extreme ultraviolet rayswhose wavelength is as short as several nanometers to several tens ofnanometers.

Next, a gate insulating film 146 which is in contact with part of theoxide semiconductor film 144 is formed. As the gate insulating film 146,a silicon oxide film, a silicon nitride film, a silicon oxynitride film,a silicon nitride oxide film, a hafnium oxide film, or the like can beformed by a plasma CVD method, a sputtering method, or the like.

Oxygen doping treatment is performed after the gate insulating film 146is formed, so that an oxygen-excess region is formed in the oxidesemiconductor film 144.

Next, over the gate insulating film 146, a gate electrode layer 148 a isformed in a region overlapping with the oxide semiconductor film 144 andan electrode layer 148 b is formed in a region overlapping with thesource or drain electrode layer 142 a.

The gate electrode layer 148 a and the electrode layer 148 b can beformed in such a manner that a conductive layer is formed over the gateinsulating film 146 and then etched selectively.

Next, an insulating film 150 including an aluminum oxide film is formedover the gate insulating film 146, the gate electrode layer 148 a, andthe electrode layer 148 b. In the case where the insulating film 150 hasa stacked-layer structure, a stack of the aluminum oxide film and asilicon oxide film, a silicon nitride film, a silicon oxynitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumoxynitride film, an aluminum nitride oxide film, a hafnium oxide film,or a gallium oxide film may be formed by a plasma CVD method, asputtering method, or the like.

After the insulating film 150 is formed, heat treatment (preferablysecond heat treatment) is performed. The heat treatment is preferablyperformed at a temperature higher than or equal to 350° C. and lowerthan or equal to 650° C., further preferably higher than or equal to450° C. and lower than or equal to 650° C. or lower than the strainpoint of the substrate. Although the timing of the oxygen dopingtreatment and the timing of the heat treatment (second heat treatment)performed after the oxygen doping treatment are not limited to those inthis embodiment, the heat treatment needs to be performed at least afterthe insulating film 150 (more specifically the aluminum oxide film) isformed. This is because, since the aluminum oxide film used in theinsulating film 150 has a high blocking effect and thus is less likelyto transmit both oxygen and an impurity such as hydrogen or moisture,release of oxygen from the oxide semiconductor film 144 can be preventedby performing the heat treatment after the insulating film 150 isformed.

Next, an insulating film 152 is formed over the transistor 162 and theinsulating film 150. The insulating film 152 can be formed by asputtering method, a CVD method, or the like. The insulating film 152can be formed using a material including an inorganic insulatingmaterial such as silicon oxide, silicon oxynitride, silicon nitride,hafnium oxide, or aluminum oxide.

Next, an opening reaching the drain or source electrode layer 142 b isformed in the gate insulating film 146, the insulating film 150, and theinsulating film 152. The opening is formed by selective etching with theuse of a mask or the like.

After that, a wiring 156 in contact with the drain or source electrodelayer 142 b is formed in the opening. Note that FIG. 7A does notillustrate a portion where the wiring 156 and the drain or sourceelectrode layer 142 b are connected to each other.

The wiring 156 is formed in such a manner that a conductive layer isformed by a PVD method such as a sputtering method or a CVD method suchas a plasma CVD method and then the conductive layer is etched. Further,as a material of the conductive layer, an element selected from Al, Cr,Cu, Ta, Ti, Mo, and W, an alloy containing any of the above elements asa component, or the like can be used. Any of Mn, Mg, Zr, Be, Nd, and Sc,or a material containing any of these in combination may be used. Thedetails are similar to those of the source or drain electrode layer 142a or the like.

Through the above steps, the transistor 162 and the capacitor 164 arecompleted. The transistor 162 includes the oxide semiconductor film 144which is highly purified and contains excess oxygen that compensates anoxygen vacancy. Therefore, fluctuation in the electric characteristicsof the transistor 162 is suppressed, and the transistor 162 iselectrically stable. The capacitor 164 includes the source or drainelectrode layer 142 a, the gate insulating film 146, and the electrodelayer 148 b.

Note that in the capacitor 164 in FIGS. 7A to 7C, with the gateinsulating film 146, insulation between the source or drain electrodelayer 142 a and the electrode layer 148 b can be adequately ensured.Needless to say, the capacitor 164 including an additional insulatingfilm may be employed in order to secure sufficient capacitance.Alternatively, the capacitor 164 may be omitted in the case where acapacitor is not needed.

FIG. 7C is an example of a diagram of a circuit using the semiconductordevice as a memory element. In FIG. 7C, one of a source electrode layerand a drain electrode layer of the transistor 162, one electrode layerof the capacitor 164, and a gate electrode layer of the transistor 140are electrically connected to one another. A first wiring (1st Line,also referred to as source line) is electrically connected to a sourceelectrode layer of the transistor 140. A second wiring (2nd Line, alsoreferred to as bit line) is electrically connected to a drain electrodelayer of the transistor 140. A third wiring (3rd Line, also referred toas first signal line) is electrically connected to the other of thesource electrode layer and the drain electrode layer of the transistor162. A fourth wiring (4th Line, also referred to as second signal line)is electrically connected to a gate electrode layer of the transistor162. A fifth wiring (5th Line, also referred to as word line) iselectrically connected to the other electrode layer of the capacitor164.

The transistor 162 formed using an oxide semiconductor has extremelysmall off-state current; therefore, when the transistor 162 is in an offstate, a potential of a node (hereinafter node FG) where the one of thesource electrode layer and the drain electrode layer of the transistor162, the one electrode layer of the capacitor 164, and the gateelectrode layer of the transistor 140 are electrically connected to oneanother can be held for an extremely long time. The capacitor 164facilitates holding of charge applied to the node FG and reading ofstored data.

When data is stored in the semiconductor device (writing), the potentialof the fourth wiring is set to a potential at which the transistor 162is turned on, whereby the transistor 162 is turned on. Thus, thepotential of the third wiring is supplied to the node FG and apredetermined amount of charge is accumulated in the node FG. Here,charge for applying either of two different potential levels(hereinafter referred to as low-level charge and high-level charge) isapplied. After that, the potential of the fourth wiring is set to apotential at which the transistor 162 is turned off, whereby thetransistor 162 is turned off. This makes the node FG floating and thepredetermined amount of charge is held in the node FG. The predeterminedamount of charge is thus accumulated and held in the node FG, wherebythe memory cell can store data.

Since the off-state current of the transistor 162 is extremely small,the charge applied to the node FG is held for a long time. This canremove the need of refresh operation or drastically reduce the frequencyof the refresh operation, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be held for a long time even whenpower is not supplied.

When stored data is read out (reading), while a predetermined potential(fixed potential) is supplied to the first wiring, an appropriatepotential (reading potential) is supplied to the fifth wiring, wherebythe transistor 140 changes its state depending on the amount of chargeheld in the node FG. This is because in general, when the transistor 140is an n-channel transistor, an apparent threshold value V_(th) _(—) _(H)of the transistor 140 in the case where the high-level charge is held inthe node FG is lower than an apparent threshold value V_(th) _(—) _(L)of the transistor 140 in the case where the low-level charge is held inthe node FG. Here, an apparent threshold value refers to a potential ofthe fifth wiring, which is needed to turn on the transistor 140. Thus,by setting the potential of the fifth wiring to a potential V₀ which isbetween V_(th) _(—) _(H) and V_(th) _(—) _(L), charge held in the nodeFG can be determined. For example, in the case where the high-levelcharge is applied in writing, when the potential of the fifth wiring isset to V₀ (>V_(th) _(—) _(H)), the transistor 140 is turned on. In thecase where the low level charge is applied in writing, even when thepotential of the fifth wiring is set to V₀ (<V_(th) _(—) _(L)), thetransistor 140 remains in an off state. In such a manner, by controllingthe potential of the fifth wiring and determining whether the transistor140 is in an on state or off state (reading out the potential of thesecond wiring), stored data can be read out.

Further, in order to rewrite stored data, a new potential is supplied tothe node FG that is holding the predetermined amount of charge appliedin the above writing, so that the charge of new data is held in the nodeFG. Specifically, the potential of the fourth wiring is set to apotential at which the transistor 162 is turned on, whereby thetransistor 162 is turned on. Thus, the potential of the third wiring(potential of new data) is supplied to the node FG, and thepredetermined amount of charge is accumulated in the node FG. Afterthat, the potential of the fourth wiring is set to a potential at whichthe transistor 162 is turned off, whereby the transistor 162 is turnedoff. Thus, charge of the new data is held in the node FG. In otherwords, while the predetermined amount of charge applied in the firstwriting is held in the node FG, the same operation (second writing) asin the first writing is performed, whereby the stored data can beoverwritten.

The off-state current of the transistor 162 described in this embodimentcan be sufficiently reduced by using an oxide semiconductor film whichis highly purified and contains excess oxygen as the oxide semiconductorfilm 144. Further, by using such a transistor, a semiconductor device inwhich stored data can be held for an extremely long time can beobtained.

As described above, fluctuation in the electric characteristics of atransistor including an oxide semiconductor film which is highlypurified and contains excess oxygen that compensates an oxygen vacancyis suppressed, and the transistor is electrically stable. Thus, by usingthe transistor, a highly reliable semiconductor device can be provided.

The methods, structures, and the like described in this embodiment canbe combined as appropriate with any of the methods, structures, and thelike described in the other embodiments.

Embodiment 6

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including game machines). Examplesof electronic appliances are a television set (also referred to astelevision or television receiver), a monitor of a computer or the like,a camera such as a digital camera or a digital video camera, a digitalphoto frame, a mobile phone (also referred to as cellular phone orcellular phone device), a portable game machine, a personal digitalassistant, an audio reproducing device, and a large-sized game machinesuch as a pachinko machine. Examples of electronic appliances eachincluding the semiconductor device described in any of the aboveembodiments will be described.

FIG. 13A illustrates a laptop personal computer including a main body3001, a housing 3002, a display portion 3003, a keyboard 3004, and thelike. The semiconductor device described in any of the above embodimentsis applied to the display portion 3003, whereby a highly reliable laptoppersonal computer can be provided.

FIG. 13B is a personal digital assistant (PDA) including a displayportion 3023, an external interface 3025, an operation button 3024, andthe like in a main body 3021. A stylus 3022 is provided as an accessoryfor operation. The semiconductor device described in any of the aboveembodiments is applied to the display portion 3023, whereby a highlyreliable personal digital assistant (PDA) can be provided.

FIG. 13C illustrates an example of an e-book reader. For example, thee-book reader includes two housings, a housing 2701 and a housing 2703.The housing 2701 and the housing 2703 are combined with a hinge 2711 sothat the e-book reader can be opened and closed with the hinge 2711 asan axis. With such a structure, the e-book reader can operate like apaper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, a display portion onthe right side (the display portion 2705 in FIG. 13C) can display textand a display portion on the left side (the display portion 2707 in FIG.13C) can display graphics. The semiconductor device described in any ofthe above embodiments is applied to the display portion 2705 and thedisplay portion 2707, whereby a highly reliable e-book reader can beprovided. In the case of using a transflective or reflective liquidcrystal display device as the display portion 2705, the e-book readermay be used in a comparatively bright environment; therefore, a solarcell may be provided so that power generation by the solar cell andcharge by a battery can be performed. When a lithium ion battery is usedas the battery, there are advantages of downsizing and the like.

FIG. 13C illustrates an example in which the housing 2701 includes anoperation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, or the like may also beprovided on the surface of the housing, on which the display portion isprovided. Furthermore, an external connection terminal (such as anearphone terminal or a USB terminal), a recording medium insertionportion, and the like may be provided on the back surface or the sidesurface of the housing. Moreover, the e-book reader may have a functionof an electronic dictionary.

The e-book reader may have a configuration capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an e-bookserver.

FIG. 13D illustrates a mobile phone including two housings, a housing2800 and a housing 2801. The housing 2801 includes a display panel 2802,a speaker 2803, a microphone 2804, a pointing device 2806, a camera lens2807, an external connection terminal 2808, and the like. In addition,the housing 2800 includes a solar cell 2810 having a function of chargeof the mobile phone, an external memory slot 2811, and the like. Anantenna is incorporated in the housing 2801. The semiconductor devicedescribed in any of the above embodiments is applied to the displaypanel 2802, whereby a highly reliable mobile phone can be provided.

Further, the display panel 2802 includes a touch panel. A plurality ofoperation keys 2805 displayed as images is illustrated by dashed linesin FIG. 13D. Note that a boosting circuit by which voltage output fromthe solar cell 2810 is increased to be sufficiently high for eachcircuit is also included.

In the display panel 2802, the display direction can be appropriatelychanged depending on a usage pattern. Further, the mobile phone isprovided with the camera lens 2807 on the same surface as the displaypanel 2802; thus, it can be used as a video phone. The speaker 2803 andthe microphone 2804 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Furthermore, thehousings 2800 and 2801 which are developed as illustrated in FIG. 13Dcan overlap with each other by sliding; thus, the size of the mobilephone can be reduced, which makes the mobile phone suitable for beingcarried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charge and datacommunication with a personal computer or the like are possible.Moreover, a large amount of data can be stored by inserting a memorymedium into the external memory slot 2811 and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 13E illustrates a digital video camera including a main body 3051,a display portion A 3057, an eyepiece 3053, an operation switch 3054, adisplay portion B 3055, a battery 3056, and the like. The semiconductordevice described in any of the above embodiments is applied to thedisplay portion A 3057 and the display portion B 3055, whereby a highlyreliable digital video camera can be provided.

FIG. 13F illustrates an example of a television set. In the televisionset, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605. The semiconductor device described in any ofthe above embodiments is applied to the display portion 9603, whereby ahighly reliable television set can be provided.

The television set can be operated by an operation switch of the housing9601 or a separate remote controller. Further, the remote controller maybe provided with a display portion for displaying data output from theremote controller.

Note that the television set is provided with a receiver, a modem, andthe like. With the use of the receiver, general television broadcastingcan be received. Moreover, when the television set is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (between a sender and a receiver orbetween receivers) data communication can be performed.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Example

In this example, properties of an aluminum oxide film used as a barrierfilm in a semiconductor device according to one embodiment of theinvention disclosed herein were evaluated. The results are shown inFIGS. 14A1, 14A2, 14B1, and 14B2, FIGS. 15A1, 15A2, 15B1, and 15B2,FIGS. 16A to 16D, and FIGS. 17A to 17D. As evaluation methods, secondaryion mass spectrometry (SIMS) and thermal desorption spectrometry (TDS)were used.

First, evaluation by SIMS analysis is described. As a comparativeexample, a comparative sample A was fabricated in such a manner that asilicon oxide film was formed to a thickness of 100 nm over a glasssubstrate by a sputtering method. Further, as an example, an examplesample A was fabricated in such a manner that a silicon oxide film wasformed to a thickness of 100 nm over a glass substrate by a sputteringmethod, and an aluminum oxide film was formed to a thickness of 100 nmover the silicon oxide film by a sputtering method.

For each of the comparative sample A and the example sample A, thesilicon oxide film was formed under the following conditions: a siliconoxide (SiO₂) target was used as a target, the distance between the glasssubstrate and the target was 60 mm, the pressure was 0.4 Pa, the powerof the power source was 1.5 kW, the atmosphere was an oxygen atmosphere(the oxygen flow rate was 50 sccm), and the substrate temperature was100° C.

For the example sample A, the aluminum oxide film was formed under thefollowing conditions: an aluminum oxide (Al₂O₃) target was used as atarget, the distance between the glass substrate and the target was 60mm, the pressure was 0.4 Pa, the power of the power source was 1.5 kW,the atmosphere was a mixed atmosphere containing argon and oxygen (theargon flow rate was 25 sccm and the oxygen flow rate was 25 sccm), andthe substrate temperature was 250° C.

The comparative sample A and the example sample A were each subjected toa pressure cooker test (PCT). In the PCT in this example, thecomparative sample A and the example sample A were held for 100 hoursunder the following conditions: the temperature was 130° C., thehumidity was 85%, the atmosphere was an atmosphere where H₂O (water):D₂O(heavy water)=3:1, and the atmospheric pressure was 2.3 atm (0.23 MPa).

As SIMS analysis, substrate side depth profile (SSDP) SIMS was used tomeasure concentrations of an H atom and a D atom in the comparativesample A and the example sample A before and after the PCT.

FIG. 14A1 shows H-atom and D-atom concentration profiles of thecomparative sample A before the PCT, and FIG. 14A2 shows H-atom andD-atom concentration profiles of the comparative sample A after the PCT,which were obtained using SIMS. In FIGS. 14A1 and 14A2, a D-atomexpected concentration profile is a calculated concentration profile ofthe D-atom existing in nature, which was obtained using the H-atomconcentration profile on the assumption that the abundance ratio of theD-atom thereto is 0.015%. Therefore, the amount of the D atoms mixedinto the sample by the PCT equals the difference between the measuredD-atom concentration after the PCT and the D-atom expected concentrationafter the PCT. FIG. 14B1 shows a D-atom concentration profile before thePCT, which was obtained by subtracting the D-atom expected concentrationfrom the measured D-atom concentration, and FIG. 14B2 shows a D-atomconcentration profile after the PCT, which was obtained by subtractingthe D-atom expected concentration from the measured D-atomconcentration.

In a similar manner, FIG. 15A1 shows H-atom and D-atom concentrationprofiles of the example sample A before the PCT, and FIG. 15A2 showsH-atom and D-atom concentration profiles of the example sample A afterthe PCT, which were obtained using SIMS. Further, FIG. 15B1 shows aD-atom concentration profile before the PCT, which was obtained bysubtracting the D-atom expected concentration from the measured D-atomconcentration, and FIG. 15B2 shows a D-atom concentration profile afterthe PCT, which was obtained by subtracting the D-atom expectedconcentration from the measured D-atom concentration.

Note that all the results of SIMS analysis in this example werequantified using a standard sample of a silicon oxide film.

As shown in FIGS. 14A1, 14A2, 14B1, and 14B2, while the measured D-atomconcentration profile overlaps with the D-atom expected concentrationprofile before the PCT, the measured D-atom concentration greatlyincreases after the PCT; accordingly, it is found that the D atoms weremixed into the silicon oxide film. Therefore, it is confirmed that thesilicon oxide film of the comparative sample has a low barrier propertywith respect to moisture (H₂O and D₂O) from the outside.

In contrast, as shown in FIGS. 15A1, 15A2, 15B1, and 15B2, as for theexample sample A in which the aluminum oxide film was stacked over thesilicon oxide film, it is found that only a slight amount of D atomentered a region close to a surface of the aluminum oxide film by thePCT and that the D atom entered neither the aluminum oxide film at adepth around 30 nm or greater nor the silicon oxide film. Therefore, itis confirmed that the aluminum oxide film has a high barrier propertywith respect to moisture (H₂O and D₂O) from the outside.

The following shows evaluation by TDS analysis. As an example sample, anexample sample B was fabricated in such a manner that a silicon oxidefilm was formed to a thickness of 100 nm over a glass substrate by asputtering method, and an aluminum oxide film was formed to a thicknessof 20 nm over the silicon oxide film by a sputtering method. Further, asa comparative example, a comparative sample B was fabricated in such amanner that after the example sample B was measured by TDS analysis, thealuminum oxide film thereof was removed, and only the silicon oxide filmwas left over the glass substrate.

For each of the comparative sample B and the example sample B, thesilicon oxide film was formed under the following conditions: a siliconoxide (SiO₂) target was used as a target, the distance between the glasssubstrate and the target was 60 mm, the pressure was 0.4 Pa, the powerof the power source was 1.5 kW, the atmosphere was an oxygen atmosphere(the oxygen flow rate was 50 sccm), and the substrate temperature was100° C.

For the example sample B, the aluminum oxide film was formed under thefollowing conditions: an aluminum oxide (Al₂O₃) target was used as atarget, the distance between the glass substrate and the target was 60mm, the pressure was 0.4 Pa, the power of the power source was 1.5 kW,the atmosphere was a mixed atmosphere containing argon and oxygen (theargon flow rate was 25 sccm and the oxygen flow rate was 25 sccm), andthe substrate temperature was 250° C.

Other three types of the comparative sample B and the example sample Bwere fabricated by further performing heat treatment at 300° C., 450°C., and 600° C., respectively. The heat treatment was performed for 1hour in a nitrogen atmosphere for each sample.

TDS analysis was performed on the four types of the comparative sample Band the four types of the example sample B fabricated under therespective four conditions (without heat treatment, with 300° C. heattreatment, with 450° C. heat treatment, and with 600° C. heattreatment). FIG. 16A shows a TDS spectrum of the comparative sample Bsubjected to no heat treatment, FIG. 16B shows that of the comparativesample B subjected to heat treatment at 300° C., FIG. 16C shows that ofthe comparative sample B subjected to heat treatment at 450° C., andFIG. 16D shows that of the comparative sample B subjected to heattreatment at 600° C., each at M/z=32 (O₂). Further, FIG. 17A shows a TDSspectrum of the example sample B subjected to no heat treatment, FIG.17B shows that of the example sample B subjected to heat treatment at300° C., FIG. 17C shows that of the example sample B subjected to heattreatment at 450° C., and FIG. 17D shows that of the example sample Bsubjected to heat treatment at 600° C., each at M/z=32 (O₂).

As shown in FIGS. 16A to 16D, it can be seen in FIG. 16A that oxygen wasreleased from the silicon oxide film of the comparative sample B whichwas not subjected to heat treatment, but the amount of oxygen releasedwas greatly decreased in the comparative sample B subjected to the heattreatment at 300° C. in FIG. 16B, and became smaller than or equal to abackground level of TDS measurement in the comparative sample Bsubjected to the heat treatment at 450° C. in FIG. 16C and thecomparative sample B subjected to the heat treatment at 600° C. in FIG.16D.

The results in FIGS. 16A to 16D indicate that 90% or more of excessoxygen contained in the silicon oxide film was released outside thesilicon oxide film by the heat treatment at 300° C. and thatsubstantially all of the excess oxygen contained in the silicon oxidefilm was released outside the silicon oxide film by the heat treatmentat 450° C. and 600° C. Therefore, it is confirmed that the silicon oxidefilm has a low barrier property with respect to oxygen.

In contrast, as shown in FIGS. 17A to 17D, substantially the same amountof oxygen was released from every type of the example sample B, in whichthe aluminum oxide film was formed over the silicon oxide film,regardless of whether the heat treatment was not performed or wasperformed at 300° C., 450° C., and 600° C.

The results in FIGS. 17A to 17D indicate that when the aluminum oxidefilm was formed over the silicon oxide film, the excess oxygen containedin the silicon oxide film was not easily released to the outside by heattreatment and the state where the excess oxygen was contained in thesilicon oxide film was held to a considerable extent. Therefore, it isconfirmed that the aluminum oxide film has a high barrier property withrespect to oxygen.

The above results confirm that the aluminum oxide film has both a highbarrier property with respect to hydrogen and moisture and a highbarrier property with respect to oxygen, and functions suitably as abarrier film with respect to hydrogen, moisture, and oxygen.

Therefore, during a manufacturing process of a transistor including anoxide semiconductor film and after the manufacture thereof, the aluminumoxide film can function as a protective film for preventing entry of animpurity such as hydrogen or moisture, which can cause a change, intothe oxide semiconductor film and release of oxygen, which is a maincomponent material of the oxide semiconductor, from the oxidesemiconductor film.

Further, the oxide semiconductor film formed has high purity because animpurity such as hydrogen or moisture is not mixed therein, and includesa region where the oxygen content is greater than that in astoichiometric composition of the oxide semiconductor because oxygen isprevented from being released. Accordingly, by using the oxidesemiconductor film for a transistor, fluctuation in the thresholdvoltage Vth of the transistor due to an oxygen vacancy and a shift ofthe threshold voltage ΔVth can be reduced.

This application is based on Japanese Patent Application serial no.2011-054786 filed with the Japan Patent Office on Mar. 11, 2011, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A method of manufacturing a semiconductor device,the method comprising the steps of: forming an oxide film; forming anoxide semiconductor film on the oxide film; performing a first heattreatment on the oxide semiconductor film; forming a conductive filmover the oxide semiconductor film after performing the first heattreatment; forming an aluminum oxide film over the oxide semiconductorfilm and the conductive film; after forming the oxide semiconductorfilm, adding oxygen into the oxide semiconductor film; and performing asecond heat treatment on the oxide semiconductor film and the aluminumoxide film after adding the oxygen, wherein the oxide film and the oxidesemiconductor film are successively formed without exposure to air. 2.The method of manufacturing a semiconductor device according to claim 1,wherein the oxide film is a silicon oxide film.
 3. The method ofmanufacturing a semiconductor device according to claim 1, wherein theoxide semiconductor film comprises indium, gallium, and zinc.
 4. Themethod of manufacturing a semiconductor device according to claim 1,wherein the second heat treatment is performed at a temperature ofhigher than or equal to 350° C. and lower than or equal to 650° C. 5.The method of manufacturing a semiconductor device according to claim 1,wherein the oxide semiconductor film comprises a crystalline region. 6.The method for manufacturing a semiconductor device according to claim1, wherein the conductive film is a source electrode.
 7. The method formanufacturing a semiconductor device according to claim 1, wherein thealuminum oxide film is in contact with part of the oxide semiconductorfilm.
 8. The method for manufacturing a semiconductor device accordingto claim 1, wherein the first heat treatment is performed at atemperature of higher than or equal to 250° C. and lower than or equalto 700° C.
 9. The method for manufacturing a semiconductor deviceaccording to claim 1, wherein the first heat treatment is performed in afirst atmosphere comprising an inert gas, and wherein the firstatmosphere is changed into a second atmosphere comprising oxygen duringthe first heat treatment.
 10. The method for manufacturing asemiconductor device according to claim 1, wherein the oxygen is addedinto the oxide semiconductor film so that a region of the oxidesemiconductor film comprises more oxygen than a stoichiometriccomposition.
 11. The method for manufacturing a semiconductor deviceaccording to claim 1, wherein the oxygen is added into the oxidesemiconductor film by an oxygen doping treatment.
 12. A method ofmanufacturing a semiconductor device, the method comprising the stepsof: forming an oxide film; forming an oxide semiconductor film on theoxide film; forming an aluminum oxide film over the oxide semiconductorfilm so that the aluminum oxide film is in contact with part of theoxide semiconductor film; after forming the oxide semiconductor film,adding oxygen into the oxide semiconductor film so that a region of theoxide semiconductor film comprises more oxygen than a stoichiometriccomposition; and performing a heat treatment on the oxide semiconductorfilm and the aluminum oxide film after adding the oxygen.
 13. The methodof manufacturing a semiconductor device according to claim 12, whereinthe oxide film is a silicon oxide film.
 14. The method of manufacturinga semiconductor device according to claim 12, wherein the oxidesemiconductor film comprises indium, gallium, and zinc.
 15. The methodof manufacturing a semiconductor device according to claim 12, whereinthe heat treatment is performed at a temperature of higher than or equalto 350° C. and lower than or equal to 650° C.
 16. The method ofmanufacturing a semiconductor device according to claim 12, wherein theoxide film and the oxide semiconductor film are successively formedwithout exposure to air.
 17. The method of manufacturing a semiconductordevice according to claim 12, wherein a peak of a concentration ofoxygen in the oxide semiconductor film into which the oxygen is added ishigher than or equal to 1×10¹⁸/cm³ and lower than or equal to3×10²¹/cm³.
 18. The method of manufacturing a semiconductor deviceaccording to claim 12, wherein the oxide semiconductor film comprises acrystalline region.
 19. The method for manufacturing a semiconductordevice according to claim 12, wherein the oxygen is added into the oxidesemiconductor film by an oxygen doping treatment.
 20. A method ofmanufacturing a semiconductor device, the method comprising the stepsof: forming an oxide film; forming an oxide semiconductor film on theoxide film; performing a first heat treatment on the oxide semiconductorfilm to remove a hydrogen atom from the oxide semiconductor film;forming an aluminum oxide film over the oxide semiconductor film afterperforming the first heat treatment so that the aluminum oxide film isin contact with part of the oxide semiconductor film; adding oxygen intothe oxide semiconductor film after performing the first heat treatmentso that a region of the oxide semiconductor film comprises more oxygenthan a stoichiometric composition; and performing a second heattreatment on the oxide semiconductor film and the aluminum oxide filmafter adding the oxygen.
 21. The method of manufacturing a semiconductordevice according to claim 20, wherein the oxide film is a silicon oxidefilm.
 22. The method of manufacturing a semiconductor device accordingto claim 20, wherein the oxide semiconductor film comprises indium,gallium, and zinc.
 23. The method of manufacturing a semiconductordevice according to claim 20, wherein the first heat treatment isperformed at a temperature of higher than or equal to 250° C. and lowerthan or equal to 700° C.
 24. The method of manufacturing a semiconductordevice according to claim 20, wherein the second heat treatment isperformed at a temperature of higher than or equal to 350° C. and lowerthan or equal to 650° C.
 25. The method of manufacturing a semiconductordevice according to claim 20, wherein the oxide film and the oxidesemiconductor film are successively formed without exposure to air. 26.The method of manufacturing a semiconductor device according to claim20, wherein a peak of a concentration of oxygen in the oxidesemiconductor film into which the oxygen is added is higher than orequal to 1×10¹⁸/cm³ and lower than or equal to 3×10²¹/cm³.
 27. Themethod of manufacturing a semiconductor device according to claim 20,wherein the oxide semiconductor film comprises a crystalline region. 28.The method for manufacturing a semiconductor device according to claim20, wherein the oxygen is added into the oxide semiconductor film by anoxygen doping treatment.